I wouldn't count on your motherboard supporting it. I had to upgrade my mobo when I'd gotten a 5870 as it "wanted" a 2.0, but my 1.x wouldn't run it, no vesa at boot. I now have a 2.1 mobo and upgraded from 5870 to 6970, now going to a 7970 and wondering if going to have the same issue as it is built for 3.0. Been almost afraid to try, the card is staring at me on the desk.
I almost bought another one today, as I want to upgrade the mobo to a 3.0 bus anyways, so waiting on that, but was curious the state of crossfire. I've been running 6 1920x1200 displays @ 11520x1200 framebuffer for probably 2+ years now, using aforementioned cards, asus ones with 6 rtc's on them and dp/dvi bus cards on their variants. I'm toying with adding a row to go 5x2 displays, thinking crossfire *might* work. Most research I've done today yesterday and today says it *can* work, but probably not with monitors on each.
Currently I can run apps like minecraft, secondlife, half-life, and team fortress on all 6 without much flaw (compiz disabled), but the 6970 supports 16384x16384 resolution, so thought to bump another row of displays, but wondering if one card can drive something like gl games on 10. I can go to ultra graphics in most without much problem windowed across all 6, but will lag if I do too much in secondlife that is probably the worst to crush video and cpu.
I'd love to know someone that actually *tried* to do this, crossfire and using rtc's on both cards for 7+ displays. I might be inclined to try if I can sell off my old 5870 and 6970 cards for another 7970. Rather not find out $2k later *no*, due to driver bugs and such a niche corner case amd will never fix it for me.
FYI, ATI's the only vendor I've seen work pumping 6 displays, let alone in linux. After years of trial and error, I found their limitations (supported framebuffer, 5xxx was 8192x8192, 6xxx is 16384x16384), and created the best desktop experience I can get with Ubuntu linux. There are quirks, the driver destabilizes over time still, but only when antagonized by compiz or games (death comes fast with both). Without GL enabled, I've gone indefinitely stable with a wrap-around desktop experience. Linux can work like this, and it's amazing for the productive, yet ADD geeks like me.
Nvidia still supports only 2 displays in native framebuffer, so they got the axe early. I wish they'd get with the program already, as their drivers were infinitely better than ati's, but at least I have my six displays with them.