if I had to choose between SLI and crossfire, I'd probabbly say SLI.
I've heard that the maximum you can have is 1600x1200 @ 60HZ using crossfire, because it uses the VGA cable to link them. the SLI bridge is much faster than the VGA cable.
and I've heard the new Nvidia drivers actually do support different cards (like 6600 + 6800) in SLI, I think they're forceware version 81.xx. they also take advantage of dual core processors - yes, video cards rely on CPU's, to make the polygons and stuff. the video card(s) just colour them.
now I'm not saying I think SLI is really great, but from what I've seen, I do prefer it over crossfire.
I'm not sure whether I think the 7800's or X1800's are better as of yet. I hope ATI does win this round, they need it after last round. because then Nvidia might start a monopoly on video cards, thus not actually try to make better cards, but get more sales. although I think having 24 pipelines is the better thing to do than boosting up clock speeds like the R520. reminds me of Netburst (Pentium 4/Xeon)