I think R.A.B. was Sirius's brother Regulus Arcturus Black (Arcturus is a family name - his grandfather's; J.K. Rowling donated a handwritten Black Family Tree to be auctioned for charity that showed this.) I believe that Regulus was named after Arcturus instead of his Uncle Alphard (while possible, I think it unlikely) because Alphard, like Sirius was an embarrassment to the Black's for being a *good* guy in a family of Dark wizards. That is what got them both blasted off the family tree. Alphard was probably always a good guy, which wouldn't inspire a Dark witch like Walburga to give her son his name. Her husband named Regulus after his great Uncle Regulus, so he probably chose the middle name in honor of his grandfather, Arcturus. That way, both names came from "admirable" Black family members.
Some of my friends disagree because of what Sirius said about his brother (he called him a "stupid idiot" and doubted that he was important enough for Voldemort to kill) but then Sirius probably discounted his power because he resented how much his parents preferred his younger brother to him. Plus, he moved out of the house as a teenager, so he wasn't there to see Regulus develop his talents (consider how much Neville improved in his 5th year). I think that Regulus discovered the concept of Horcruxes, aside from being a Death Eater, he was from a family of Dark wizards. Voldemort has wanted to become immortal, so anyone with knowledge of Horcruxes could assume that he'd create one. He had Kreacher help him steal and then hide the locket Horcrux before he was killed. So Kreacher will be important in Book 7. His danger to the Order was underestimated in Book 5, and I think that so far, they've also underestimated how valuable he could be too. Harry can order him to tell him what he knows about R.A.B. and the locket (which I think we already saw at the Order's HQ in Book 5). Kreacher had retrieved several items that Sirius tried to throw away, I'd bet the locket was among them. That is if Mundungus hasn't stolen it, in which case it is probably at Borgin and Burkes.
Snape is such a complicated character it is difficult to know where his loyalties lie, but my theory is that he is only really loyal to himself - willing to do whatever he has to to get what he wants. He didn't get to be Head of Slytherin House by being a wimp, he is clever, cunning and ruthless. Like Draco, he went to Hogwarts with preconceived notions about blood purity and the value of people with different beliefs. Remember, he knew more about the Dark Arts as a first year student than most seventh year students. That sort of fascination speaks of someone that seeks power for self-protection and/or control of others; which isn't something that would concern a typical eleven year old. Snape was probably tired of feeling helpless and determined to learn all that he could to make sure that he never felt that way again. From his childhood memory and nickname, I'd say that his Muggle father disapproved of young Severus and his mother using magic. To make things worse, he would've had uncontrolled outbursts of it as a kid, especially in times of stress. The teasing and bullying that Snape tolerated while at Hogwarts, his unattractive appearance, and lack of friends, caused him to focus on his talents. He'd want to prove them all wrong, that he was worthy of their respect. I can understand the appeal that the Dark Arts and Voldemort had, offering him a way to go from ridiculed to feared.
Snape is clever enough to have quickly figured out after joining him, that Voldemort is actually a sociopath that won't hesitate to torture or even kill his followers if they disappoint him or attempt to leave his service. So, then he was stuck, unable to simply quit and walk away, but I believe that he was looking for a way out. From what he overheard of the prophesy, he might've suggested to Voldemort that Harry was more of a threat because he thought Neville (the Pureblood) was the Chosen One and didn't want Voldemort to kill the only one that had the power to stop him. That and he hated James Potter, of course. Either way, after Harry survived the attack, Snape knew that his best chance to escape a life spent at the mercy of Voldemort would mean carefully assisting Harry, so that he could fulfill the prophesy and kill Voldemort, and thus free him of his obligation to him. If Harry is sucessful, Snape can admit his role in making that possible and finally be acknowledged as the hero he is. If he fails, then everyone knows how much Snape has hated Harry and he can continue in his role of a "faithful" Death Eater.
Now about the Horcruxes, Voldemort wanted a 7-part soul, which would mean creating 6 Horcruxes, with the 7th part of his soul being the one within his body. The diary and ring have been destroyed, and I've already said I think that the locket is at #12 Grimmauld Place. I think the last three are Hufflepuffs' cup, a relic of Ravenclaw's and a relic of Gryffindor's. We know that Voldemort liked to use significant deaths to create his Horcruxes with, so I think that he intended to use Harry's death (not his body) to create his 6th and final Horcrux. But, since he didn't kill Harry I don't think he was able to make a Horcrux that night. He wouldn't have considered James or Lily's deaths worthy, they were only obstecles in his way to his goal: Harry. Of course that would mean that he'd taken an object with him to the Potter's home to use. I'm betting that would be the Gryffindor relic, which would be poetic because James and Lily were Gryffindors that were hiding out in Godric's Hollow. I think Peter Pettigrew led Voldemort to the house and was there to collect his wand and the unused relic afterwards - both items that he returned to him since.
Voldemort probably created his 6th Horcrux with the murder of Frank Bryce, the Muggle caretaker that worked for his hated Muggle relatives (father and grandparents). It was stated that Frank had continued to care for the property as an employee of the new wealthy owner. What if that wealthy owner is Lucius Malfoy? He'd been trusted with the diary and the retrieval of the prophesy. Also, Voldemort killed the Riddle's in that house as a teenager, and it served as his hideout before he got his body back so the place has significance for him. He probably hid the Horcrux within it. I think that Ravenclaw's relic was the tiara that Harry saw when he was looking for a hiding place for the Half-blood Prince's potions book within the Room of Reqirement. That would be a fitting hiding place for it since Ravenclaw is credited for designing the ever-moving floor plan at Hogwarts (according to her Famous Witches and Wizards card). Not many people are aware of the location of the room and even Dumbledore wasn't sure how to access it. I think most stumbled upon it accidentally and as it was full of damaged, illegal, or contraband items - those who had used it to hide those objects wouldn't likely share it's location with anyone. Voldemort had time to discover the room as a prefect, then Head Boy, plus he lived at Hogwarts year round. He could've hidden the Horcrux there as a student or during his two visits afterwards when he applied for the Defense Against the Dark Arts teaching post. Hufflepuff's cup is probably hidden within Hogwarts too, possibly within the Chamber of Secrets. Hogwarts is supposed to be an even safer place to store a valuable item than Gringotts after all.