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.