Screaming Penguin - Source Does Matter

HardOCP has the D3 Benchmarks straight from id. The short version: Buy NVidia -- mostly because they have tweaked their drivers specifically for the next generation id engine.

His high holiness says:

It should be noted that all of the modern cards play the game very well, and benchmark scores should not be the be-all-end-all decision maker. Scores will probably improve somewhat with future driver releases, and other factors like dual slots or dual power connectors can weigh against some of the high end cards.

The benchmarking was conducted on-site, and the hardware vendors did not have access to the demo before hand, so we are confident that there is no egregious cheating going on, but it should be noted that some of the ATI cards did show a performance drop when colored mip levels were enabled, implying some fudging of the texture filtering. This has been a chronic issue for years, and almost all vendors have been guilty of it at one time or another. I hate the idea of drivers analyzing texture data and changing parameters, but it doesn't visibly impact the quality of the game unless you know exactly what to look for on a specific texture. On the other hand, the Nvidia drivers have been tuned for Doom's primary light/surface interaction fragment program, and innocuous code changes can "fall off the fast path" and cause significant performance impacts, especially on NV30 class cards.

A note on overclocking: it is very likely that overclocked configurations that "play everything else perfectly" will start to show problems on D3 due to new usage patterns. Everyone is of course free to do whatever they want with their own hardware, but don't complain to us...