I've currently got a setup that is using a single MSI GTX 660 Ti Power Edition, and although I am okay with all my games now, I am planning on upgrading to a SLI setup in the future.
My current system has 3 main fans, one at the back to draw heat out of the system (1950RPM), one at the top to blow cool air onto the CPU/Mobo/GPU (1950RPM), and one at the front to supply the bottom of the case with cool air (1200RPM).
At an idle state, my GPU's average temperature is around 25-28 degrees, which isn't too bad, but I was thinking about the implications of SLI and overall case temperature. My current plan is to add an extra fan at the bottom of the case (just beside the PSU) to take in cool air and blow it straight in to the GPU area. Would this provide sufficient cooling?
Finally, I've read that the reference cards move air to the back of the GPU, would it be better me getting a reference 660 Ti as the second card? If not, would the two MSI cards stacked on top of one-another cause any heating problems?
Idle temperatures are useless. Graphics cards used heavily can go up to 70 C or 80 C (sometimes higher but only on high end cards where there is a lot of power involved) without any issue. Most cards in your level will not reach that heat level, i'm guessing during game play you probably get to 50 C MAX, so two cards would raise the max temps to somewhere between 60 and 70 so you would be perfectly fine. I would recommend you make that top fan pull air out of the case, and if possible, get another bottom intake and you should be good to go. I would just go with another MSI Power edition if I were you.