The revision only applies to the PCB, not the core components like the GPU or RAM modules. I've both and they are roughly equal in terms of over-clockability. As far as the difference is concerned, you only need to look at both cards to notice. The Rev C appears to have a higher grade of ceramic and the traces are much more polished. If I was to guess, I would say that either Visiontek refined their PCB process or they've contracted another outside vendor for this part (or vice versa).