My 8800gt sli costs far less than and equals the performance of a gtx280. SLI is available on so many cards that whether dual setup is for you depends on a ton of parameters.
I had an SLI mobo and an 8800gt w/22" 1680x1050 monitor. I could not max out a number of games with AA i.e. assassin's creed, lost planet, Crysis (still), CoD4. So I could spend $140 on another 8800gt, or $500 (at the time) for a gtx280. Easy choice that. Even with the price drop to around $400 it still didn't make sense for me.
Now I can max out every setting including AA on every game, and likely all the upcoming games (Far Cry 2, and Crysis:Warhead if they come thru on the promised optimizations).