VGA charts IV: ti4200-8x VS 6800GT.
Call of Durty:
ti4600:69.9
6800:99.6
6800GT:143.0
As far as I know, the 6600GT is slightly slower than the 6800 - thus, the ti4600 is 30% slower than the 6600GT in DX8 games.
Comparing in DX9 games is meaningless since the ti4x00 series are DX8.1 GPUs (fixed shader functions).