These are probably your best bang for buck options which should set you up for a good few years.
http://www.newegg.com/Product/Produc...82E16814261100
That's what I've got and it every game I have, including some brand spanking new ones, it maxes with some speed. Even though it's combined with a shitty Athlon 2 quad-core. Your benefits of going NVidia are PhysX (example video at the bottom) and the availability to upgrade to stereoscopic 3D if you ever fancy it (ATi don't really have a good solution for this yet)
http://www.newegg.com/Product/Produc...G-_-14-102-948
http://www.newegg.com/Product/Produc...s-_-14-102-948
The comparable ATi card. With this, you'll get similar performance with less power drain, runs cooler and slightly cheaper, but you do miss out on the bells and whistle from NVidia (although you do get eyefinity, which is an awesome multi-monitor set-up). not sure on the differences between the two above, apart from one's cheaper and one comes with 2 games.