I'm a firm supporter of ATI ever since I bought a mach64 card for my old computer and witnessed massive performance increase (this being 1997, and the old card was a 1Mb Trident 9440AGI), without the stability problems of the GeForce card I tested in the system just beforehand.
I know that the GeForce series have got much better, but I also know that my favourite games have compatibility problems with their later drivers. My Radeon 9700 Pro has had NO COMPATIBILITY PROBLEMS WITH ANY GAME, and I've had it since Christmas. Except for Bedlam, but that's so ancient even my old card wouldn't run it properly (why the hell does my mouse pointer disappear when it gets more than a quarter of the way down the screen in that game?)
Now all I need is an 8x-compatible motherboard, since it's a pity to have such a high-end video card without the motherboard and processor to take advantage of it. Are there any plans for 16x AGP yet?
I've heard good things about the FX5900, but I'm not sure how it compares to the Radeon 9800 Pro (which requires slightly less extensive cooling).