I know this is late or whatever but I have the same mobo with crossfire 5770s and I see double the performance from one card. I know x16 x4 isn't ideal but for higher resolutions it makes a world of difference.
But are you using a SATA3 (6Gb) SDD drive at the same time? Otherwise you will at least avoid the problem that lmeow is mentioning.
(I'm replying since I'm in exactly this conundrum and contemplating wither it is worth running CrossFireX on this board or if I should bite the bullet and buy a new one.