You want a GeForce 8600 256meg or better to get a solid 60fps. Lower spec cards can manage 60, but it'll drop rapidly if flames are involved.
If you need a half-height card ATI's are easier to find, but their drivers suck for GL & the past few releases have repeatedly broke TB shader support, so I can't recommend them.
Maybe upgrade to a middle class card. If your computer's motherboard and power supply can support it I'd suggest a Nvidia GT 240 or 250..
I will destroy you for recommending that..... the GT 240 is "slower" than the 9800GTX+ and is much more expensive, the GT250 is still slightly lower than the 9800GTX+
For effect: DONT BUY A 240 or 250!