sli, in your case seems rather hard to recommend as you would have to purchase a new mobo, case and possibly a new psu too, you're talking to spend 300-350$ on your comp only to fit an extra 150$ piece of hardware in it, that doesn't really seem efficient money spending at all.
I get the idea that spending an extra 150$ can get you to the near gtx580 performances but it only works in some selected games only, well most of the big commercial release.
The first problem with that idea is power consumption, the duo will burn 320w of power this mean that you need a 650w psu minimum or in the 700+ ideally, with 4 6pin connectors (100$+).
The second problem is that 320w is a hell lot of heat and your gonna need a way of getting that heat out if you don't want to cook you pc, the first solution would be to play with your case open with a house fan blowing into it or purchasing a performance oriented case ( way over 100$).
The third problem, micro stuttering, it's caused by varying time lapse between frame, some people are more sensitive to it than others, also the degree of the stuttering vary with different games.
your best solution would be to purchase an hd 7870 (350$), it consume only 15w more than a gtx 460, it has performances nearing the gtx 580 in every game ( not just the sli/cf certified games only), no micro-stuttering, no mobo upgrade, supports DX11.1 ( latest revision of directx - 460 doesn't)
