id Software's John Carmack's latest .plan update outlined some general performance and image quality differences between NVIDIA's NV30 (GeForce FX) and ATI's R300 (Radeon 9700Pro). Here is a copy of the pertinent details of that update:

NV30 vs R300, current developments, etc

At the moment, the NV30 is slightly faster on most scenes in Doom than the R300, but I can still find some scenes where the R300 pulls a little bit ahead. The issue is complicated because of the different ways the cards can choose to run the game.

The R300 can run Doom in three different modes: ARB (minimum extensions, no specular highlights, no vertex programs), R200 (full featured, almost always single pass interaction rendering), ARB2 (floating point fragment shaders, minor quality improvements, always single pass).

The NV30 can run DOOM in five different modes: ARB, NV10 (full featured, five rendering passes, no vertex programs), NV20 (full featured, two or three rendering passes), NV30 ( full featured, single pass), and ARB2.

The R200 path has a slight speed advantage over the ARB2 path on the R300, but only by a small margin, so it defaults to using the ARB2 path for the quality improvements. The NV30 runs the ARB2 path MUCH slower than the NV30 path. Half the speed at the moment. This is unfortunate, because when you do an exact, apples-to-apples comparison using exactly the same API, the R300 looks twice as fast, but when you use the vendor-specific paths, the NV30 wins.

The reason for this is that ATI does everything at high precision all the time, while Nvidia internally supports three different precisions with different performances. To make it even more complicated, the exact precision that ATI uses is in between the floating point precisions offered by Nvidia, so when Nvidia runs fragment programs, they are at a higher precision than ATI's, which is some justification for the slower speed. Nvidia assures me that there is a lot of room for improving the fragment program performance with improved driver compiler technology.

The current NV30 cards do have some other disadvantages: They take up two slots, and when the cooling fan fires up they are VERY LOUD. I'm not usually one to care about fan noise, but the NV30 does annoy me.

I am using an NV30 in my primary work system now, largely so I can test more of the rendering paths on one system, and because I feel Nvidia still has somewhat better driver quality (ATI continues to improve, though). For a typical consumer, I don't think the decision is at all clear cut at the moment.

For developers doing forward looking work, there is a different tradeoff -- the NV30 runs fragment programs much slower, but it has a huge maximum instruction count. I have bumped into program limits on the R300 already.  As always, better cards are coming soon.

As is often the case his comments left us with more questions. John, however, was gracious enough to answer a few choice picks from those raised by Beyond3D forum members.