yeah they are compatible but your crossfire would only run at x8 speeds for both instead of x16.. if you're gonna get two 4890's, im assuming you're gonna be running high end graphics..
also, if you're getting that mobo, im assuming you're running an amd cpu.. with these assumptions, i should tell you that you should look for a board with a 790fx northbridge chipset (instead of your 790x).. the 790fx should run your 4890's at x16 speeds for no bottleneck..
for psu, look into the following brands: corsair, antec truepower new, and pc & power cooling
if you read the other threads, you can get a better idea of what you might need...
2 4890's = massive heat.. you should look into a case with good air circulation..
Yeah your motherboard is fine and will work with two 4890s. Of course a board based on the 790FX would be preferable, especially since the cost of them has come down alot, but if you already have the Gigabyte GA-MA790X-UD4P then you don't need to replace it. Anyway as far as 850W units you should look at