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..
for motherboard, maybe look into these options:
http://www.newegg.com/Product/Product.aspx?Item=N82E16813136044 cheapest which uses ddr2 ram
http://www.newegg.com/Product/Product.aspx?Item=N82E16813136067 uses ddr3 ram
http://www.newegg.com/Product/Product.aspx?Item=N82E16813128377 if you like gigabyte (ddr3 ram)
also look into the asus m4a79 series motherboards or the asus crosshair iii