You're way ahead of me on this issue. Hey, maybe you (or another reader) can fill me in on something: If I'm correct on this, graphics cards get firmware or maybe software updates that often significantly speed them up. Has this card come out with any fixes or updates? How often do these happen? Are the HD 2600 Pro and HD 2600 XT running slower than the older Geforce options because they haven't been maximized by the manufacturers yet?
Firmware for GPU's doesn't improve or detract from performance. What you are looking for is updated drivers. Drivers dictate how the software communicates with the hardware. ATI and Nvidia update their drivers every month Apple doesn't update their drivers that often. This is due to the increases in performance are only seen in games.