I don't know the full specs of a 360 image and your PC, but I would at least recommend upgrading to a 550 or 560 Radeon. (50-80 bucks at Fry's Electronic). Seems to play the latest games fine even on Proton/Steam and it seems to manipulate images very quickly. Try that out and see how it goes first before upgrading anything else.
A lot of image editors these days depend on GPU. So that should help speed up Gimp. I personally just pop open the case anyway, and get the motherboard model. That way you know what RAM/CPU you need to be looking for when upgrading.