How much do you want to spend anyways ? For 3d apps, the single best card you can get is probably the nVIDIA GTX 275 that costs $200-$250 depending on brand. If you want something cheaper then a GTX 260 will also do ok for $150-$200. Cheaper still you can maybe find older cards like the 9800GT or 8800GT. Avoid models like the GTX 285, GTX 250, 8600, 9400 etc as they are bad value for money.

Personally i've had bad experience with ATI so i wont suggest those, but they also have comparable cards in the same price range.

Also note that all decent modern cards require beefy power supplies to run so you may have to upgrade that as well. A 650w+ PSU from a good brand like Corsair/Antec/Coolermaster etc will be what you want to get.