There are two main reasons for this:
1) The 8800GT maxes out the FPS capacity of your processor in 3dMark06 a bit more than your 320 did.
2) The 8800GTS320 (and all the G80 cards, really) reflected % increases in overclocking with % fps increases more linearly than the g92 products generally do. Personally I suspect this comes from a combined effect of already-higher clocks with a lower ROP count vs the g80 cards. Lower memory bandwidth may also come into play in some cases, at least to a small degree.
What you're seeing is not at all uncommon, bottom line. Having an overclocked core 2 duo or quad would manifest itself in larger changes to your score based on your overclock, because as I say, you are going to be getting some CPU bottlenecking on that test with your processor and an 8800GT.
Edited by brettjv - 6/18/08 at 7:20pm