so a single 7850 would be better than those in sli/crossfire?
Yes. SLI/Xfire is helpful at higher resolution and with AF/AA cranked up. With SLI/Xfire comes stability issues and a phenomena known as micro-stuttering. Granted SLI/Xfire has come along ways and is much more stable now days. I run a GTX 590, using SLI all the time without issues for the most part, and before that I had GTX 570's in SLI. I have 3 monitors and do Widescreen gaming on occasion, so SLI is a must to crank out 5760x1200 and a good framerate.
I always advise and build systems with the single best GPU that fits in the budget. if a year down the road more GPU power is needed, you can SLI/Xfire for a speed boost or when you see a big price drop for you second card.
A pair of HD 6850's will beat the 7850, but will cost a few $ more and use 2.5x the power (electric bill) and create 2x the heat and noise.
GTX 480 would also be a recommended card at it's current price on the egg at $229.
Outperform both the 7850 and GTX 560Ti.
Once overclocked it can surpass the GTX 580.
Downfall is power consumption and in some cases heat.