A problem with Arthurian legends is their extreme variability, far more different from one another than most surviving versions of Greek myths.
For example, there are a number of medieval stories of the Grail Quest, and some of them scarcely have an incident in common with others. Even the hero varies, in one version it is Gawain, in most versions the hero is Perceval, and in the latest versions the hero is Galahad son of Lancelot, backed by Perceval and Bors/Boort, who is Lancelot’s cousin.
Geoffrey of Monmouth’s “Historia Regem Britanniae” is the earliest surviving attempt to provide a somewhat complete history of Arthur, and most accounts of Arthur in medieval chronicles follow Geoffrey. See http://en.wikisource.org/wiki/History_of_the_Kings_of_Britain .
The story starts with Arthur defeating the Saxons, then beginning some foreign conquests, first Ireland, then some Scandinavian countries, and then Gaul, where he defeats Frollo, tribune and ruler of Gaul, in single combat. Finally Arthur clashes with the Roman Emprire, fighting against an army of eastern and western troopa under the command of one Lucius Hiberius, Procurator of the Roman Republic. Arthur is also said to have slain two giants, and one of these battles is very fully told. After defeating the Romans, Arthur learns that his nephew Mordred whom he has left behind in Britain as regent has taken his queen Guenhumara as wife, and is in rebellion against him. This leads to a final battle in which most troops on both sides are killed, Mordred himself is slain, and Arthur vanishes, supposedly gone to the fairy isle of Avalon to be healed of his wounds.
The verse Arthurian romances mostly don't have much to do with this, but are stories of adventures of individual knights set vaguely in an Arthurian milieu. However at least some writers were interested in combining the Arthurian conqueror tradition of Geoffrey with the romance traditions.
The first such surviving cycle consists of three romances, the “Joseph of Arimathea” by Robert de Boron, the “Merlin”' by Robert de Boron, and a romance of Perceval probably by someone else that completes the story. See http://www.amazon.ca/Merlin-Grail-Arimathea-Perceval-Attributed/dp/0859916162/ref=sr_1_9?ie=UTF8&s=books&qid=1218850730&sr=1-9
Robert de Boron is very concerned with the Holy Grail. His “Jospeh of Arimathea'' tells of the origin of the Grail. His “Merlin” provides what looks like Geoffrey’s story of Arthur’s father and uncle and grandfather in a form that seems to have entered oral tradition. Some form of the story of Perceval seems to be expected, though the version that we have has so many disagreements with the first two tales that most think it is not by Robert de Boron. After Perceval achieves the Grail, we have an account of Arthur's conquest of France, his war against Rome, and war with Mordred, and his final disappearance as found in Geoffrey, but simplified as though it too has passed through oral tradition. See http://answers.yahoo.com/question/answer?qid=20080814140811AA5fJ1X chapters I-VI for an early English translation of Robert de Boron’s “Merlin”, and see http://members.terracom.net/~dorothea/baladro/index.html Prologue 3 to the first part of chapter 18 for a modern English translation of a somewhat modified medieval Spanish version of Robert de Boron’s “Merlin”. For the “Perceval” connected to this cycle see http://www.everything2.com/index.pl?node=Didot%20Perceval&lastnode_id=1076107 . Note that you can click on and read the “Mort Artu” section at the bottom of the page which tells of Arthur’s conquest of France, his war with Rome, and his final battle.
Essentially this cycle follows Geoffrey’s account closely, but adds to it the story of the Grail as the central event of Arthur's reign.
The later Vulgate Cycle is what many readers think of as the standard version, the version in which Lancelot and his love for Guenevere becomes central to the story. There are three earliest works in this “Vulgte Cycle”: the voluminous “Prose Lancelot'' and two shorter sequels, “The Quest of the Holy Grail”' and the “Death of Arthur”. Both these last two are available in English translation from Penguin.
The story starts as a biographical romance of Lancelot but turns into a story of the Arthurian world, as Lancelot becomes more and more important to that world, at least in this version. Arthur’s battle with Frollo for the throne of France is again told, but occurs almost as an accident during the main story, which tells how Lancelot and Arthur regain Lancelot’s homeland (which is presented as being in Anjou in France). The grail is also introduced. It is prophesied that Lancelot’s son Galahad (who appears for the first time in the “Prose Lancelot” so far as we know) will achieve the Grail. Perceval, though present, is rather downplayed. In the Quest of the Grail, more is told of Galahad, Lancelot himself, and Lancelot's cousin Bors/Bohort than of Perceval. In the “Death of Arthur” it is not a war on Rome but a war against Lancelot that leads Arthur to cross the sea and leave Mordred behind. Though an attack by the Romans occurs when Arthur is across the sea, it is over in two paragraphs. After Arthur’s disappearance, we have an account of an attempt by Mordred's sons to gain the kingdom, but while in Geoffrey they were defeated by the new King Constantine, in this account they are defeated by Lancelot.
This cycle was then extended by adding the two works of Robert de Boron as sort of a prologue. But the “Joseph of Arimathea” disagreed in a large number of places with the early grail history as appeared in the main cycle. Accordingly the “Joseph” was later replaced by a new work called “The History of the Grail”. And the “Merlin” was lengthened by an additional sequel giving any early history of Arthur’s kingdom, with Saxon battles and a fuller account of the Roman war than that in the “Death of Arthur”. An incomplete alternate version of this sequel is also found. This full cycle was called “The Arthurian Vulgate Cycle” by its first editor H. Oskar Sommer, the word “vulgate'' here meaning “common, ordinary”, referring to it being considered by a large number of medievals as the most normal form of the Arthurian legend. Later works such as the “Prose Tristan”, the “Post-Vulgate Cycle”, the “Prophecies of Merlin'', and “Guiron the Courteous” largely accept the account as told in the Vulgate Cycle, save for the Merlin sequel which is ignored. They sometimes actually include parts of the Vulgate Cycle, sometimes with minor modifications, and often refer to incidents that occur in the Vulgate Cycle.
They add to it, expecially adventures concerning Tristan (who is only mentioned once in the Vulgate Cycle itself), Lamorat who is Perceval’s elder brother, Palamedes the Saracen (who becomes important in the Grail quest, and Blioberis (a cousin of Lancelot),
The Vulgate Cycle can be found on the web at http://www.archive.org/details/arthurian01sommuoft . Just edit the number at the end of the word “arthurian'' from “01” to “08” for each of the eight volumes in the box at the top of your browser. An English translation of all but the alternate Merlin sequel was available as “Lancelot-Grail”, edited by Norris J. Lacy. This can now be obtained from some stores in used editions. Or you can still get an abridged version: http://www.amazon.com/Lancelot-Grail-Reader-Selections-Arthurian-Humanities/dp/0815334192 .
“Le Morte d’Arthur” by Sir Thomas Malory is the best known English version, mostly compiled rather haphazardly from the Vulgate Cycle and later works partly based on the Vulgate Cycle. But Malory had some very good material, and at least towards the end of his work, is a magnificent writer. Accordingly, despite its incompleteness and other defects, “Le Morte d’Arthur” became the standard legend of Arthur for English readers so that modern novelists and retellers are far more likely to invent material to fill into for what is not found in Malory rather than look it up the material in older medieval works. Unfortunately Malory apparently had little access to the “Prose Lancelot”, only reproducing a few unimportant sections for late in the story and some sections he found inserted in the version of the “Prose Tristan” he was using. Accordingly in his account the story of Lancelot lacks its beginning, just as Malory’s story of Palamedes lacks its proper end as it would appear if Malory had the complete story.
Those who claim it is all found in Malory show themselves to be very ignorant of the breadth of Arthurian legend, and ignorant of its lows and highs. If in some cases Malory is arguably better than his sources, in some cases he is arguably much worse. But Malory adds realism to his account, without throwing out any of the fantasy.