I recommend either a GeForce2 GTS 32mb, GeForce GTS 64mb, or GeForce2 MX 32mb.

The MX will cost less out of the other two, but it is not as fast. All of these three are 'decent' cards. However, the GeForce2 GTS 64mb will play games now a days at high resolutions and high detail with hardly no loss of speed.