xthekidx :
BFG is not a company that is known as a good PSU manufacturer. Corsair has a reputation for good PSU's that last a long time, BFG does not. That PSU is new, you have no idea how long it will last. Going with the single rail PSU that is manufactured by a better company makes a lot of sense to me. $10 is a small price to pay for extra assurance that your PSU will not explode and take the whole system with it.
You realize that BFG and Corsair don't actually handle the manufacturing of the power supplies right? That's done by Enhance and Seasonic respectively. Just jump on the popular brand band wagon, and spew fanboyism out of your mouth. I don't even want to flame anybody, its just, you're shooting out such inflammatory language, the OP is going to shy away from anything that isn't 'reputable'.
In all honesty OP, if this were a few years and you wanted to build a system, xthekidx would be telling you the same thing about Corsair not being reputable because they are new to the power supply market, contrary to early testing that shows they have a good product. Either the BFG LS-550w or Corsair VX550w will be great power supplies for you to get. I just feel at this point in time, that BFG is offering a better product with a better warranty than the competitors.
The build I suggested to you is a great option, for your price range. Is there anything you want to discuss about it? Possibly some things you want to switch out? I know gkay09 mentioned a Biostar 790GX board, and that's a good place to start. Some people might scream Asus at you and tell you they are the best motherboard maker on the market, but look to the benchmarks for that. The 4th core unlocking that he was talking about comes from a BIOS option called ACC, advanced clock calibration. Setting it to auto can unlock the 4th core the Phenom II X3s, however, there's a reason it was disabled by AMD in the first place. You might be lucky and have a working 4th core for a free, which is awesome, or you might experience some stability issues.
I don't know if anybody answered your questions about OS choices. I'd hate to steer you one way or the other. $100 on an OS isn't cheap. What's more painful, is that Win.7 is going to be coming out very soon. The RC is slated to come out in April if I recall, but you wouldn't really want that as your main OS in the mean time. Win.7 is looking very appealing, and I'll probably be picking up a 64bit copy when it hits the streets. I skipped Vista entirely, because I wasn't going to dump another $100 on an OS after just buying XP.