Any reasonably new monitor should work with either SLI or crossfire. I think that DVI connectors give the best image quality, but there are a few people that think an old style analog VGA connector can be tweaked into a better image than DVI. The only thing I've heard is that above a certain resolution (I don't remember what the cutoff numbers are) you might need a monitor capable of "DVI-D" (dual link DVI). But you wouldn't want to run an LCD montor above it's physical resolution anyhow, and you can reasonably expect that a high resolution LCD will have the right connector type to run it at it's maximum resolution.
So bottom line, there is no special monitor required to run SLI or Crossfire, however a DVI connection should give you the best image quality.
One other consideration with SLI and crossfire however, is that it makes no sense to run SLI and/or crossfire at a low resolution. Spending $800 on an SLI setup and then using a small monitor with what is now considered a fairly low resolution of 1280x1024 just doesn't make sense unless you're just running benchmarks and not actually *using* the computer for anything. Any 22" to 24" widescreen LCD should be perfectly capable of letting you get the most out of an SLI or Crossfire setup, and some of those are not terribly expensive compared to the cost of either SLI or Crossfire setups. If you're going for SLI or Crossfire with cheaper video cards though, you should consider going for a faster single-card configuration since a single one of the latest generation video cards typically beat 2 previous generation cards working together, with less hassle and lower power draw.
