What resolution and settings? And are you talking about 3dmark06? At 1280x1024 (default benchmark), that's about average. The 8600GT is not really a high-end card. An 8800GT scores approximately twice that in 3dMark06.
That sounds ok for an 8600 GT. Don't beleive the ratings you may see for "similar machines" on the 3D comparison sites. A lot of those machines are way over clocked to get those ratings. Your machine should be clumped in the middle of the ratings with everyone else.
Also it depends on which 3D you are using. Some weight very heavily for Video card and others for processor.