From what I know of the pairing of APU and GPU has not reaped great rewards. Some feel AMD dropped the ball on the graphics pairing for Richland and now Kaveri.
With that said I would get a R9-270x. It is above the cheap entry range but not high powered like the 280x and 290s. The 270x alone will outperform the APU/GPu pairing you thought of doing. The best part is you can add another 270x in the future if you need more GPU power. A pair of 270x can perform on pair with a 290x. See here CLICK ME
The prices for the 270x fluctuates wildly so keep an eye on and pounce when the price drops again. Sometimes you can catch a ref model for $199-209. Other times it will be as high as $280!