well, I was trapped on the nvidia bandwagon, until some of you guys injected some ATI into my blood
Well, with the top-end cards, just go and look at any benchmark comparing the FX5950 with the Radeon9800XT or Pro and you will be stupid to get an FX. Radeons produce better AA and AF image quality, perform better in with AA and AF enabled and are beaten only in a few games where resolution is set to standard with no Anti-Aliasing or Anisotropic Filtering, but the Radeons hold out better when there are alot of tasks to perform at high resolutions and of course, the DirectX9 compatibility issues with the FX cards, Radeon is king! The only problem with the high-end Radeons is that the FX cards overclock alot better than the Radeons, but clock speed doesn't make up for compatibility issues in the chip.
For the mid-range cards, I would say that the FX5600ultra cards are about even with the Radeon9600ultra cards. With the mid-range cards, total all-out performance and image quality isnt 'as' important as with the high-end cards, so that is why the FX has a chance to shine abit more.
But for high-end cards, stick with the Radeons, as I eventually decided to get the Radeon9800XT over the FX5950ultra.
And with the difference between an 8X card in an 8X slot and an 8X card in a 4X slot, there isn't really much difference. Only about 1-2% or no more than 5fps difference in the higher cards.