To give you an idea, here is a comparison between the 8600GT and 7600GT:8600GT v 7600GT. In most cases the 7600GT wins or comes close in performance. Based on that you could assume that the 7600GS would be equal or slightly inferior to the 8600GT. If you could spare the money, an 8600GTS or X1950 would be better options for only a little more money. That's only the case if your PSU can support either of the cards I mentioned.
8600gt is a great overclocker.... get it then same or even better than the stock 8600gts because they r the same build card