Lots of great cards for under $100 these days(HD4850 deals have been $80-85 lately)
A case w/acceptable psu should be $50ish
500 gig HD $50
4 gigs of ddr2 can be had for $30-40 and below($20ish if you wait for a nice deal to come around)
A $70 e5200 c2d can be easily overclocked like crazy and is good for a budget gaming system. An AMD Kuma 7750 is $60 and offers similar performance although it doesn't OC quite as well.
An ok motherboard for the e5200 should be $50-70. Slightly less if you go for an AMD system.
That's all the main components for under $350. A dvd burner is $20ish... wireless card if you need one about the same. The system I just outlined should play any game quite well at reasonable resolutions, even Crysis. If you need a monitor too things get a bit more pricey and if you'd rather spend more for a quadcore system add another $50-100.