Err.. the term for AMD is crossfire, not SLI. I think first you should check whether your motherboard support it. And if you're able to buy two 6850, why not go for a single card but higher spec such as 7870 or 7950?
Good point! Realized that myself after thinking about it a bit. Did some research too and buying just one higher spec card is better then two lower specs running simultaneously. Thanks again!