I have an 8600M GS, too, and I also have problems with low FPS (not as low as 5 in general, but when i get to even lightly populated areas i get as low as 2).
This infuriates me, as I have an nvidia gf ti4600 with an athlonxp2600+ in a desktop that blows this framerate out of the water.
(Laptop is a core 2 duo 2ghz, 2gb ram, gf 8600M GS)
In response to the 'ram usage' post, my 4600 has 128M video ram and 1GB system, while my 8600 has 256M Dedicated video ram, and can use some of the 2gb of system ram. There should be absolutely no issues with that at relatively sane settings.
Also, I, too, am using vista, and have tried a few different drivers. Over the next few days I'll be trying more, and I'll report back on how they work out :|