Depends on your budget, power supply, and what kind of slot do you have (i.e., AGP, PCI, PCI-E)?
If you have a PCI slot, go for a Geforce 6200.
For lower end computers with power supply of 250W, go for 9550 Radeon or 9600XT Radeon.
For higher end AGP and PCI-E, go for Geforce 6600 + or 1600XT ATI and above.