DOOM 3
The X600 is not very playable at 1024. At 1600, the 6600GT has roughly a 50% advantage in framerates compared to the X600 at 1024.
Here is a graph of the 6600GT only. There is virtually no difference in framerates on the 6600GT when AF is enabled. 1024 with 4xAA yields good, playable framerates. This is an impressive feat that the 9800XT and 5950 cannot hope to acheive.
Image Quality
6600 GT
Left Column No AF, 4x AF, 8x AF (No AA)
Right Column No AF, 4xAF, 8xAF (4xAA)
X600
Left Column No AF, 4x AF, 8x AF (No AA)
Right Column No AF, 4xAF, 8xAF (4xAA)