Fobby, I have a 8600 gts in my old rig. It will not play renX, so this is why.
You should get something over a 8600. A 8800 GT OC, or a 9xxx, or the newer 3 digit series over 220.
Troll http://dealnews.com for a few weeks for a good deal on a graphics card. That's how I got my evga 250 GTS for less than $80.