The 6600GT goes for about $150, but that card is getting slowly outdated. I would definitely get a 6800GT in AGP because that's the best card AGP supports. It's also a very good card and it will last you a while. They are a bit hard to find though... because they're slowly fasing out AGP to make way for PCI-E.
AMD 64 3200+ Newcastle@ 2.2Ghz
1GB (1x1GB) OCZ Performance Series
ATI X700 Pro 256MB OC\'d 475/525
OCZ Modstream 520W
Samsung 914v 19\" 12ms LCD
SB X-Fi w/ Logitech Z 5300e