Dudes, there isn't a single card out there that can deliver the performance of SLi. Of course, referring to games that are supported by SLi. I'm excluding the one's that aren't supported for they are irrelevent to this discussion.
In SLi, you can play with all eye candy maxed, a res of 1600x1200 with 4xaa and 8xaf and still get over 100fps for your average frame rate. In Doom3 I believe you'll still get near 140-160 fps. NO single card can compare to that. Now, if you look at that SAME SLi rig a year from now, it will STILL get better fps than a single X800. Add another year and you STILL get better fps than an X800. (this even goes with the X850XT PE). So, in the long run- if you DON'T want to have to upgrade for awhile, SLi is NOT a bad choice. Sure, it IS expensive- possibly TOO expensive, and YES- it IS over rated, but it will offer longer lasting performance than ANY single card out there today.
SLi MAY die off, eventually. I think it will. But NOT for a LONG time. Since ATI is now in the SLi game, game devs will be programming with THAT in mind, dual card designs.
Yes, I know that they must also take into consideration the folk that DON'T have SLi or AMR rigs, but the devs will make the "options" and "enhancements" available to those whom have such rigs.
HL2 is already being re-vamped:
Games like F.E.A.R. will continue to push graphics even higher, so if you want longevity and you have the cash for SLi, single card is NOT the best route.
Show me ATI's card (besides the R520, that isn't out yet so it is irrelevant) that can come close to dual 6800GTs with visuals maxed, 4xaa/8xaf @1600x1200- last I checked the best AVG. frame rate for the X850 XT PE was around 80fps, peaking at and slightly above 100fps. So, no- single card IS NOT the best choice for longevity IF you have the cash for SLi. Or even AMR for that matter.