that's strange. i was using Painter 8 (without the update) on my new PB (1.25GHz, 512MB RAM) and it was running fine it did slow down a bit after i'd been using my compy for 5 days without rebooting though. 😛
you should also check out some other forums. if i remember correctly, over at conceptart.org forums there's a forum about Painter. you might find some answers there.