If I needed a cheap card, I know I would take a Radeon HD 6570. But be careful, it is a cheap card, so you shouldn't expect wonders.
(Need at least a Power Supply of 300W)
It should be enough to play games with project64 1.6 and the default plugins (and Glide64). Some very demanding games like Perfect Dark may run slow.
If you're willing to pay a little more, a Radeon HD 7770 is far better but :
-Need at least a Power Supply of 450W
-might be rapidly bottlenecked by your CPU
edit : no Nvidia there of course, cheap and Nvidia don't go together well
Thanks I'll check my power supply and buy a new GPU