You need to consider a few things:
1. At the moment, contrary to what AMD say, Eyefinity doesnt perform that well on crossfire setups. It seems to "just" work. I have to disable crossfire to get playable frame rates.
2. Some games dont run well on Eyefinity. Here is a list that "do":http://www.hardocp.com/image.html?image=MTI1NDE2Nzk2NjBpOFVuVWt6c0RfMV8xNl9sLmpwZw==http://support.amd.com/us/eyefinity/Pages/eyefinity-software.aspx
Thanks to paperfox for the links.
3. FOV issues in FPS applications such as MW2. There is a fix on widescreengaming.com but it doesnt seem to work in my case.
4. You will have to turn down settings (AA/AF/Post processing) to get good frame rates. You may even have to run in Directx9 to run.
Having said that, Eyefinity is fantastic. Hopefully driver support will eventually come out, hopefully. For FPS, I use a single screen (motion sickness on Eyefinity!) while on flight sims/racing I use Eyefinity.
As vvhocare5 mentioned, beg/borrow a couple of monitors and try it first. You may need an ACTIVE displayport adapter first (~$100).