well theres different opinions
Ati is:
the best in the market (x800 xt platinum edition)
is expensive
uses one slot
uses less energy
Nvidia's are:
better deal (GeForce 6800 GT)
use a little bit more power
usually uses 2 slots (depending on manufacturer)
more performance for your money
i would recommend the Nvidia 6800 GT, cuz the X800 Pro is about the same price as the GT, but it has 4 pipelines closed, so the GT has 16 pipelines and the X800 Pro has 12 pipelines. Thats why when it comes to more bang for you buck, Nvidia owns Ati.