Putting a 3dfx card in there would reveal a whole new ("old") world for you;
There aren't any new drivers for these cards since.. uh.. 2000
And, it won't run any games that require TnL with adequete performance.
Id'e say, go to your local PC shop, and get a GeForce FX 5200 (NOT SE/XT!!!!) or a Radeon 9200 (again, NOT SE!!!).
Both of which will give you some excellent bang-for-buck
(and they cost around 50$)
If you're willing to spend a bit more (100$?), get the Radeon 9600 (yet again, NOT SE!!!), or if you manage to find one; a Radeon 9500Pro.