Can this board run 2 3870x2

yes

It has two PCIe 2.0 slots, so sure it can run Crossfire. However, there's a caveat with CrossfireX. A single 4850 is as fast as a 3870x2 without Crossfire issues (i.e. if a game has lousy Crossfire support, it only recognizes one of the GPU's on the card, not both). Four GPU's on two cards is CrossfireX, and support has been spotty. You may not find the improvement to be worth it.

I know, I thought of it too. I thought if I got a better 790 board with Crossfire support and a second MSI 3870x2 that I'd get great performance. The cards are cheaper now than when they first came out, but you'd probably need an 850 watt PSU to have the right number of six and eight PCIe connectors. Overall, I'd say the performance boost isn't all that great at most resolutions.

Here are some reviews of two 3870x2's in CrossfireX:

http://www.octeamdenmark.com/forum [...] php?t=1228

http://www.maximumpc.com/article/h [...] crossfirex

http://www.anandtech.com/showdoc.aspx?i=3256&p=5

Older reviews, with the lastest from June, but I haven't seen anything detailing recent drivers improving things. The first review gives about 16 fps difference with two 3870x2's in Call of Juarez and the second review mentions around 20 fps in Call of Duty 4 with two 3870x2 vs. a single. Not the best improvements for the cost of entry.

Even if you have a good enough power supply, I'd recommend waiting till June 2009 and getting of of the next ATI cards to arrive (leaked as the 5xxx series). That's what I'm doing, waiting 8 months or so and skipping both CrossfireX with the 3870x2 and the 4xxx series (where the 4850 is just as fast and the 4870 only slightly faster).

However, if you have the cash and the interest. It would be nice to see benchmarks with newer drivers for CrossfireX. I'd be curious to see if the drivers have matured, but I'd think the newer drivers are aimed at improving the 4xxx series. The drivers that work best for me in the games I'm playing with a single 3870x2 is Catalyst 8.7 (8.9 caused black screens of death in LOTRO for me, so I switched to the older drivers).