In the GIMP preferences dialogue System Resources, you can enable Use OpenCL under Hardware Acceleration.
Whether or not OpenCL will work for you depends on how good your OpenCL environment is. It’s not always the easiest thing to get working, and a lot of support depends on proprietary drivers. That’s the case with Linux anyway, I’ve no idea how Windows things go in the OpenCL world, or how well GEGL is able to access them on that platform.
On the GIMP side of things, some GEGL filters have working OpenCL ports, as in they can work with OpenCL. Where they do, the speed up is very worthwhile. So, you are faced with a situation where many parts of GIMP’s background processing will still be processor bound.
For now I think, getting a really fast CPU is going to be the best bet. At least until:
- Your platform has a solid OpenCL implementation.
- All or most of GEGL is able to make use of that OpenCL implementation.
Good point about canvas drawing. I think go for the best CPU, but get a system which is going to allow you to add a good GPU later on.