Well cooling is fine, the cyclone cooler was meant for the gtx 560, and the 6850 doesn't get as hot as that. and If that becomes a problem I can buy aftermarket coolers.
The games I play that I "need" crossfire for I'm certain support crossfire. I.E. Crysis 2, Battlefield 3, Skyrim...Mainstream high graphic intensity games like that. Games that aren't like that my single 6850 can handle.
My psu/motherboard supports crossfire.
600w is enough?
Micro-stuttering is perfectly true, and ESPECIALLY for 6850 since it's lower-end than 6970 (for which it's less bad). As you mentionned, SLI performs better and does have less micro-stuttering, that is a fact. Some games won't even support it, and even when the fps is higher, it often won't feel smoother. Between two 6850 and a 7850/70 which could be about the same price, I wouldn't hesitate to go for 7850/70. In your case, I'd just wait and get a worthy single-gpu upgrade in the future.
Con: using two graphics cards usually requires more power and therefore a beefier power supply. You should check that your power supply provides enough PCIe auxilliary power connectors before you attempt to crossfire your 6850. Each requires one 6 pin PCIe power connector.