nobody knows, wait until both dx10 cards are at least out.....
or better yet, wait till dx10 even comes out
if you want to buy something now, SLI's hands down the winner, because you can get better cards for it
the next few months are going to answer your question, tho
i'll be waiting, myself
as of now, tho, SLI beats crossfire hands down. it's all going to come down to the performance of ATI's cards.