The 8600gt is a PCI-e card so pretty much any PCI-e card under the sun will fit.
What card you choose depends on your budget and what you want to do with the pc. When buying a video card you have to keep in mind that the larger ones have large power needs so you need to check that the PSU can handle the card.
As Pro-g suggested the 9800gt is a great, cheap all purpose card. Tiger has one for around $75. If you wanted some thing a little better I would suggest a AMD 5770-6770 (they are the same thing). Both of those cards have a modest power requirement of around 100watts so you should check to be sure your PSU has the capacity.