The BEST you can pair that APU with is an HD6670, and it's called "Dual Graphics" when using an APU + Discrete card.
The R9-270X is a lot more powerful than the iGPU in the APU you have. I don't have any benchmarks handy, just this: http://www.hwcompare.com/15688/radeon-hd-6670-oem-vs-radeon-r9-270x/
As said, just install the new card, hookup the monitor to the discrete card. I'm not sure if the iGPU is disabled automatically or not. If not, you may have to disable it prior to adding the discrete card.
Update to the latest AMD drivers.