I kind of doubt that SLI will become that popular. Seems like too much of a hassle to have two cards. Also, having two of anything doubles the probability of something going wrong, cuz if one hits the high road the whole system doesn't work.
I'm honestly satisifed with the high end nvidia and ati cards anyway. I honestly don't think I will be needing an upgrade for a while. I'd rather companies just produced good games instead of worrying about this proverbial pi$$ing contest about which game has the prettiest graphics. Doom3 and HL2 weren't *that* fun, I've played a lot of amateur games that cost nothing to make with relatively bad graphics that were more satisfying.