its probably a little high for you, but i just ordered a ATI X850PL for $350 from Tigerdirect!!!!! that is an amazing price.
anyway I would to AMD especially because you want the most bang for your buck (who doesnt really? -so everyone should get an AMD) that will save you some money on your ram too, withought loosing proformance. there should be a dual core 3800 on newegg. Maximum pc said the amd dual cores are better, and I beleve them, they have good extensive benchmarks. although if your mostly going to be gaming, that dual core wont help you out much at all! Infact unless you really think you need it for other things you may do (video editing, alot of intensive photoshop) I would definatly say that dual core is not worth it. just get a faster single core processor, wether AMD or Intel. Look for a single core AMD to save you some more money.
Get a cheap case. aspire, or something not so tricked out and trick it out if you want yourself.
I would say the ATI card, but I dont have any spicific data to back that up.
also, check your sunday adds in your newspaper. I remeber back when it would have been $120 on newegg i found an 80 gig HDD at bestbuy for $40 with rebaits. Also found a 52x cd burner for $20 a year ago. and a dual layer DVD drive for $40 about 4 month ago at Office Max. generally newegg is best, but sometimes...