Look at this http://www.neoseeker.com/Articles/Ha...a6800gs/4.html
This shows that the GT performs slightly better on high resolutions and with AA. But that doesn't include all games, if you look at COD2 you will see that the GS performs the same or better on all settings; and in FarCry they perform just about the same.
Now look at the overclocking with the GS. Thats a huge boost in performance from OCing, overclocking usually only give 3% increases on stock cooling from what I've seen and hear but this gives an extra 12 fps in 10 X 7 in Doom 3. He really should OC, but that is up to him.
Even without OCing it performs about the same and better in some cases for $40 dollars cheaper. The GS is the way to go.