Official, AMD suggests to crossfire it to a 6670 at the most. But benchmarks and real world performance shows that that isn't really as good as buying a standalone GPU and using only it. A 6670 will cost you around 65 to 70$, I would recommend just getting a 7750 or 7770 GPU. Here is why:
http://www.tomshardware.com/reviews/silent-pc-gaming-performance,3435-14.html
The 6670 crossfires and works well but even a 7750 beats it without the integrated GPU.
If you don't have more than 65$ to spend then get a 6670. If you want more power, I would go for a 7750 or 7770.
Cheers!