8600 GT's will be better. Note that there are two versions of the 8600gt flowing around. One has DDR2 memory and therefore clocked lower while the other has DDR3. The performance difference is pretty significant, about 500+ 3dmarks(06) between the two. I believe there are only two notebooks that use DDR3 8600gt's, the asus g1/g2 and the macbook pro. The dells, sager 2090's all use ddr2.
Also note that while both versions are fast then 7700's, the 7700 doesn't lag too far behind the ddr2 one.