There are a ton of 512Mb 7950 AGP cards on ebay for around $100.
Alternatively, you could get something more up to date (but not nvidia) if you go with a Radeon 4650 or 4670:
The 7950GT is a pretty decent card though. Not real fast by todays standards but still respectable enough to play some games. A 7900GT will run Crysis at medium settings, and a 7950 is faster than that.