it is the GPU, in all honestly it's slower because it is a nvidia and it's quite an old one at that. If you want to upgrade, from what I understand AMD cards are the best to go with. However if you want to stick with nvidia and are willing to upgrade at a reasonable price. I'm getting about 90Mh/s with a GTX 560ti from EVGA, not the best, but definitely better than what you're getting right now.
Well I run 6 monitors off of my main desktop and don't want to take a chance of screwing that up by changing out video cards. It was tough enough to find a combination that worked well together. So maybe I will do some research and try to find an ATI card that will work in my old machine. However, it doesn't even support PCI Express and would probably have to be PCI or AGP, so that may not leave me with any good options...too bad.