4890 and 5850 crossfire capatible

Why not buy another 4890?

Unlikely as one is DX11 capable and the other is not, easier to EOL the DX10.1 card and move on.

stolen from mousemonkey at:
http://www.tomshardware.com/forum/271370-33-crossfire-4...

no they are not compatible for crossfirex. 4890 and 5850 belong to two different card series, batches, generations and so on. amd allows only the same class cards from one gen to cfx e.g. 6870 can cfx with 6850, 6950 can cfx (tri-fire) with 6990. if the model number is 'abcd', then the crossfire compatible cards must have 'ab' numbers matching. one exception is 5770 being able to crossfire with 6770 as the 6770 is a rebadged 5770.
if this is your motherboard, it supports only x16+x4 cfx mode. you'll lose quite a bit of gfx bandwidth on the x4 slot, resulting in poor performance from the second card.
http://www.newegg.com/Product/Product.aspx?Item=N82E168...
also check out this cfx/sli faq and the compatibility chart in the thread:
http://www.tomshardware.com/forum/245454-33-crossfire-f...