I recently retired my 8800gts 640 SSC (the special edition one with 112 sp like the 8800gt). It was a great card, but was starting to struggle a bit at 1920 x 1200 as I couldn't run COD4 maxed out at that rez. However it ran COD4 @ 60 fps vsynched great at 1680 x 1050.
The GTX 260 is quite a bit faster -- max settings at L4D I get near 70 fps minimum now everthing maxed @ 1920 x 1200, whereas my 8800 got 58 avg and ~ 41 fps during big zombie rushes...and that was only with 4xAA/4xAF. Can't wait to see how this card handles WIC, which I basically had to put aside when I bought my LCD.
Long story but the point is, if you need > 1680 x 1050 resolution or are planning on buying a 24" LCD, go with GTX 260 or 4870/4890 at a minimum. Below that, you could probably get away with a $100 card.