My Dell 730 originally came with my two cards set up with Crossfire. I've tried both using Crossfire and independent cards using two 24" widescreen flatpanels, but really didn't notice a difference either way. I do use Keyclone and have inactive windows capped out at 15 FPS, so I'm probably not stressing the cards very much either way.

I guess it all comes down to how you are going to use them. One big monitor? Use SLI/Crossfire (as if this wasn't obvious enough). Two monitors? Then probably either way would work fine, but you'll be able to tweak performance for each card/monitor more easily with independent cards and using 3rd party utilities. Three or more monitors? Then you should plan to keep the cards independent and run multiple monitors from each.

Just my two cents, but I'm far from an "expert" in this area.