I would definitely go with Win7 (64-bit). No matter what - go 64-bit with a new PC. Only reason to stick to 32-bit is if you have some old hardware or software that's not 64-bit capable. Anymore there just isn't much of that going around.

I'd also concur on the 9550 vs 9400 - for $30 more it's a nice upgrade.

I've never been a big fan of getting expensive power supplies. I generally stick to the generic ones (like rosewill). Still 500 watts can go pretty quick, if you see a 600 watt one for $10 more, then go for it. Personally I still have my $480 watt PS and it's running fine with about everything you have listed here except I have 3 hard drives. Maybe I've just been lucky that I've never been bit by going with a cheap PS though

As for your motherboard, I would definitly look at something a little better. 4x RAM slots are pretty much a required item for me. 4GB of RAM will get you by for now, but if you want to do any multiboxing beyond 2 chars then I'd definitly go with 8GB. But so long as you have 4x RAM slots and 2 of them are empty, you can always upgrade that later.

Unless you are going with some pretty large overclocks I disagree with getting a seperate CPU cooler. You can even overclock a little bit with the stock heatsink if you want. Total waste of money IMHO. If you want to spend an extra $80 to get a higher clock speen - then get a better CPU in the first place (core i7 920) and run that one with the stock heat sink.