ArtRage doesn't really use multi-processors - it's pretty-much a single-threaded application. We decided that the overhead in ensuring thread-safety wasn't going to give as good a performance boost in the general case than just writing a more efficient system.
So your best focus for improved performance is a faster single-core or dual-core processor (your OS is quite happy to be doing stuff in the background using multiple processors).
Pressing <F5> to switch off the lighting renderer in ArtRage can make a significant speed improvement. (Unlike other painting applications, ArtRage applies directional lighting to your canvas so your paint strokes have depth and texture)
AndyRage's mantra for graphics engine code:
"Sure - how hard can it be?"