It is not so much the video card these day as your CPU and your ping times from your internet connection. Anything 2.8 GHz and above and a higher than a 9800 series vid card is overkill, and won't actually help you in online gaming all that much. SLI definitely won't help at all, and IMHO is a waste o' cash. You are not going to be able to tell the difference in frame rates between one 6800 and 2x 6800 cards. You might be able to crank up your detail levels to max without paying too great a penalty, but my 9800XT allows me to do that with FarCry now (even the water at ultra high detail).
Then of course there are people like me who like to push computers as hard and high as they will go! :^)
DO NOT get a PCI Express board yet. There are still bugs to be worked out, and DDR2 memory is expensive, and not yet as fast (latency-wise) as DDR1. You will not get any benefit from PCI X just yet. Programs aren't that fast yet. BTX will be out by the end of this year, and that will drive down the cost of ATX parts.
Thanks for the info Dude.