For online first person shooters, it's been nice for me. Some maps use more resources then others do. 75% of my kills in cs:s are from hs's, you can't make those running on a low resolution, too blocky to get one off repeatedly. I went from a 9800pro, to a 6800gt, to 2x6800gts with cs:s, and every single time i get lag i notice a big difference. I run at 1280x1028 in cs:s, and i still get lag on one or two maps. Everybody else bitches about it too, but they talk about 3fps while i'm sitting as low as 20fps in the same area. 20 looks like a slide show to me, i don't know how people deal with running slower systems.
With other games...It hasn't been as big a deal. I can run my 9800 system or my 6800 systems with no problem in something like EQ2 (so long as i have plenty of ram...), or WoW. Don't get me wrong, EQ2 has some seriously intensive graphics, but i can turn them all on, or leave 60% of them off, and barely notice a difference. It doesn't affect gameplay.
I say nay to the naysayers! SLI is just a more powerful system... I would just buy the highest end single card you can afford... then add a second before it goes out of stock. Now it doesn't matter which brand you use, so long as they're the same gpu. Of course it may make more sense to wait until next Dec, and purchase a new m2 system (am2 whatever...prices will have dropped a little by then from June).