DislikedI have been doing a side research on this too , I currently have a double monitor config using a single nvidia card , which is kinda old , and thats all the motherboard can support
as I plan to make it 4 monitors system, it needs 2 gpu and the motherboard needs to support crossfire (for ati) or SLI (for nvidia) to connect the 2 cards
so my suggestion is to have ur graphic-card decided first , and then buy a suitable motherboard that supports either sli or crossfire,
I prefer having Nvidia , I had ati radeon (9800) which burns twice in 2 years...Ignored
You do not need SLI or XFire for multiple monitors. If you want to hear a sad bit of news, SLI video cards only support a single monitor output anyways, instead of the four +2 TV's I should be able to. Ati is coming out with a new tech that will help alleviate this issue called EYEfinity. It allows for massive screen stretching.
For everyone else living here on earth that just wants to expand their graphics space, just buy a dual head PCI graphics card. They're like $30-40 and will support two monitors easy peasy. Otherwise, if you really wanted to, you could look into getting an NVS420 from nvidia which supports four monitors per single card.
It's really not as hard as everyone thinks it is. It's also a lot cheaper than everyone thinks it is!
Edit: I personally have an SLI setup, so I'm limited to a single display. That said, I haven't really noticed a difference either.