Go 64 bit if you can, preferable linux since it's usable right now out of the box. Slam as much ram into the machine as the motherboard(make sure you MB will allow 8 GB or more memory) will allow and you can afford. If your stuck with Windows only, then make sure to get 64 bit version of windows. There is a 64 bit GIMP beta version (does not support python extensions in the install ).
The only issue with 64 bit stuff is legacy and come custom device support. I have not had a problem with most devices, but my son's action replay will not installed since I assume it does not come with a 64 version of the driver needed to transfer data.
Remember, at the very most, 32 bit Windows can give a bit less than 2 GB "tops" to a single program and available memory tops out around 3.2 GB for your all your apps and the user mode OS stuff. The last .8 GB is reserved for device drivers and the OS Kernal and stuff....