To the OP: whichever one is cheaper. Both will be unsuitable for modern gaming, and I doubt you'll be able to tell the difference between 10 fps and 11 fps. Neither card is designed for eye candy at > 1024x768 or higher resolutions and difference in performance should be under 10% in most titles, with the ATI card being the zippier of the two.
The 8600GT is more linux-friendly and cooler running, while the 2600XT with correct software will offload more Blu-Ray DVD playback processing under Windows. Features and price should be the only things considered when looking at entry level gaming hardware like this, not performance.
Don't be such a tightwad, spend the extra $20-30 on four times the card. You'll be sorry if you don't.