I'm currently running on the 920 and I'm having the same issues however my CPU is overclocked to 4.0Ghz and I'm running the 6870. MiRai has been helping me work through the upgrades so I can effectively box 8 and reading that thread may help you with your decision. http://www.dual-boxing.com/threads/4...r-8-x-accounts. I can 8 Box at the moment and have yet to try 10 however the 8 box is a bit sketchy with this system. That's not to say your current video card isn't your limiting factor but that upgrading may not be enough to effectively run 8 in DX11 at low settings, I still need to try DX9.

Windows 7 Professional x64
i7 920 2.66 Ghz; Overclocked to 4.0 Ghz
Corsair Hydro h50
Corsair M4
AMD XFX HD6870
Corsair CMPSU-650TX 650-Watt
Patriot Viper Xtreme Series DDR3 24 GB (6 x 4 GB) PC3-16000 2000MHz
GIGABYTE GA-X58A-UD3R