Ya you can. You can Crossfire any 2 cards from the same series. To xfire a x2 and a reg card you need Vista/Win7. You can find the info here:http://support.amd.com/us/kbarticles/Pages/ATICrossFire_comp_req.aspx
Someone tested the 3870x2 with a 3870 in xfire back in 2008 here:http://www.xtremesystems.org/forums/showthread.php?179665-My-Short-Time-With-Hybrid-Crossfire-3870X2-and-Regular-3870.
By now it should be
considerably more stable.