PSU Question on New Build

I'm building a ~$500 system since that's all I can manage at this point. My question (first question) is what max wattage should I get for a PSU? I've got my system cost at $520 (including shipping and taxes), but would like to save as much as possible.

System:

ASRock N68PV-GS Mobo (May change to ASUS if I can drop some costs elsewhere)
AMD Athlon 64 X2 5200+ Brisbane
4G (2x2G) G-Skill PC2 6400 mem
WD 320G SATA HD
MSI N9600GT OC GeForce 9600 GT 512MB
Pioneer 22x DVD+/-R Drive
HEC 585w PSU
Generic Rosewill case (I don't need anything flashy. Just need good airflow)
Vista Home Premium 64bit (I know I know. But it's only $99 and I can upgrade to 7 later)

I've tried a couple of PSU calcs, but nothing seems that up to date. But what I've seen is something along the lines of 324w needed at peak load. I want to make sure I've got enough power, but not overkill as I'm sure my current choice is ($ reasons too).

The other question I have is, is there really a noticeable difference between the AMD 5000+ Brisbane and the AMD 5200+ Brisbane? Again, looking to shave cash if possible.

Any helpful hints would be appreciated.

Quick Edit: This system is for a gaming rig, but I do not go for the latest and greatest in games usually. The most recent purchase I have made was Left 4 Dead. My base games that I usually play are L4D, CS:S, TF2, and BF2. So obviously those aren't the most power hungry of games.