8800GTS is way better. You forgot to look at the core. The 8600 uses the G84 core, while the 8800 uses the G80 core. The G80 is much better even though it has a lower clock speed.
Think of the 8800 as a Core 2 Duo, and the 8600 as a Pentium 4. A 2.4 GHz Core 2 Duo would blow away a 3.4 GHz Pentium 4. Same with the 8800 vs. the 8600.