I think the title pretty much is self explanatory. I'm wondering if down the road it would be better to just buy another 7850 to go along with the one I have (will have) or to just bump up to a GTX 670 graphics card?
a single 670. you'll at least don't have to deal with possible issues like drivers for newer games (this goes for Nvidia as well, so i'm not dissin' AMD here), microstutter, etc.
it may not perform as good as 2x7850 but the 670 should be sufficient until you feel like getting another cheaper 670 to run in SLI somewhere down the line. unless you REALLY wanna max out every games in 3 x 1080p monitors
SLI/CF issues aside, 2x7850 now would perform slightly better than a single 670 in some cases.
a single 670 can still handle 3x1080p monitors with reduce settings though. add another 670 later on when the prices drop and it will beat that CFed 7850 unless you seriously want to add another card or two for a 3-4 way 7850 crossfire.