If you want to use GIMP and python, your out of luck with Windows on a 64 bit. I did not have a problem just running GIMP on Win64, but the installer does not have python support. At the end of the day, I dual but to Ubuntu and have been using Ubuntu almost exclusively for 6+ months. Of course, GIMP 2.8 is due out fairly soon and may actually have full 64 bit support.

My biggest suggestion is to get as much RAM as possible. I have 6 GB and was amazed how much better GIMP performed on the same map(lot's of layers) once I switched from 32 to 64 bit and the increased RAM threshold. Granted, the processor was umm like 9-12 months newer, but I still believe that the RAM was the major differentiator. For example, turning off a layer on the map I was was working on took 15-20 seconds (or more some times) on the old machine, while the new machine it took .5 seconds or so....