Look for the GeForce 4 Titanium 4200. They are tough to find, but a great deal at around $160 - $200. (They will outperform all but the GF 4 Ti 4400 and 4600.) I do not recommend any of the GeForce 4 MX line, they are not well priced for what they offer. The GeForce 3 Titanum 200 would be a good card in the $100 to $150 range, and I'd recommend these over any of the MX series.
The ATI Radeon 8500 card is probably not too bad either, but to be honest I've stayed away from ATI because of some bad experiences in the past. Supposedly their Radeon drivers have improved recently though, and image quality and DVD decoding built in is generally a plus for the Radeon series.