You could get a StealthXStream 600W PSU if you want a really good video card and your PSU holds you back. I'm suggesting it because it's pretty good quality, cheap, and I actually fit one in an HP once. PSUs like 520HX or Silencer 610W wouldn't have worked in that smallish case. Maybe yours is different, I don't know. What size is your current PSU (length, width, height)?
How much clearance do you have now, with the 8600GT? That is, if the card was 4 inches longer, would it still fit? The best video cards available these days are almost 11" long, and IIRC the 8600GT is under 7".