Need help with frame-rate issue!

Hi, I recently upgraded from a nVidia 7900GTX to an Radeon HD 6670 GDDR5 graphics card and I was having some frame-rate issues with the Radeon that I did not have on the 7900.

While I can actually play some games that I couldn't before with the new card, iv'e noticed a frustrating loss of FPS in Diablo 3 for example.. I see some cool looking new special effects with the Radeon that I didn't before (which is cool).. but it seems to be reducing my frame rate uncomfortably.

I was wondering why the performance took a hit with a supposed upgrade to my graphics card. Iv'e reduced all the special effect settings to minimum (which I didn't have to do with the old card) and lowered the resolution.. as well as using the Catalyst Control Center to lower the settings to increase performance.

Has anyone else experienced what feels like a "downgrade" with an upgraded graphics card and is there options or settings that I can use to optimize my graphic cards performance?

System Info:
Operating System: Windows 7 Home Premium 64-bit (6.1, Build 7601) Service Pack 1 (7601.win7sp1_gdr.120830-0333)
Language: English (Regional Setting: English)
System Manufacturer: Gateway
System Model: FX7026
BIOS: Default System BIOS
Processor: Intel(R) Core(TM)2 Quad CPU Q9300 @ 2.50GHz (4 CPUs), ~2.5GHz
Memory: 4096MB RAM
Available OS Memory: 4026MB RAM
Page File: 2010MB used, 6041MB available
Windows Dir: C:\Windows
DirectX Version: DirectX 11
DX Setup Parameters: Not found
User DPI Setting: Using System DPI
System DPI Setting: 96 DPI (100 percent)
DWM DPI Scaling: Disabled