Is micro-stutter still a big problem with AMD cards? I know my friend who ran a pair of 4870x2 had no end of issues with that. So much so that while his average FPS was better and his bench scores were far better, games played smoother on my then mid-range GTX 275. Since then he's upgraded to a pair of GTX 480's and really hasn't had problems of that sort since. I'm getting a 2nd GTX 570 myself to go SLi and don't anticipate too many issues based on my friends 480 SLi experience. Note: he always had more than ample power for the cards he ran - running two PSU's even.
I know micro-stutter used to be considered more an ATI/AMD problem (from what I've read) but really thought the latest hardware/drvers had buried this problem for the most part.
I'd imagine that if you have card X, then getting a 2nd card X will always be a cost-effective solution to get a big FPS boost. This is assuming that card X is reasonably current gen of course.
Note: I've not had and ATI/AMD GPU in my system for a while, so I'm just going on my friends experience.
Cheers,
Scoob.