It's a GX750? The younger brother of that one, the GX650, was described by HardOCP as "a polished turd in a box." I read the review, and it may be ok up to around 80% of its label. Yours can probably handle an upgrade, but I would not assume it is cleanly capable of any more than 650W.
Whether a CPU overclock would help you depends on where your current bottleneck (all systems have one, by definition) is. If your system really slows down when you raise graphics quality settings and/or resolution, but speeds up when you lower them, then the graphics card is the bottleneck. If changing those settings doesn't seem to make much difference, then most likely your CPU is.
Replacing the motherboard (an Asus Sabertooth or ROG board will overclock well) might only be worthwhile if you discover the CPU is the bottleneck. Remember that a new motherboard typically also means a new copy of Windows, although Microsoft may be willing to assist you to reactivate over the phone when you explain that you replaced your motherboard (but they might not, so be prepared).