How do the Nvidia 8600GTS & GTX 260 video cards compare?

I have dual SLI 8600GTS 512mb cards in my comp now, but am considering an upgrade to the GTX 260, but only one card. I'm wondering what the comparison would be. Would it be better just to stay with my SLI config, or buy the GTX 260, possibly wait to buy one more? I need help, thanks :)|||The 360 will wreck your current setup. However, your SLI config will work for now, so I suggest waiting, and then getting two 260's.