Yes, it will work.
The architechture in the new APU's gpu is Cape Verde, Cape Verde includes the 7730, 7750, 7770, R7 250, and R7 250X, no others.
All of the above cards will work in dual graphics with the 7850K, the 7790 and 260X WILL NOT as they are bonaire architechture.
All 7000 series and R7 and R9 series are GCN, doesn't mean they're all crossfire compatible.