Im thinking about SLI. I too have heard mixed reviews on it. But Newegg accidently sent me 2 video cards (Lucky Me) of the same type shown in my signature. I have a mATX motherboard that has 1 PCI-E X16 slot. So I would have to get a new case, motherboard, and cooling because my card runs hot when overclocked... It's pricey, but I met a guy at a flea market selling computers and He told me it really did pay out.
I guess im really going to have to experiment my self, I think it is the video cards that you SLI, not SLI it self. You can expect to get ripping performance with a 7200GS. But you can expect wonders with 8800GTS. I guess it really comes down to parts.
In the future im going to try SLI.