More RAM, don't ya' know?
I've used both PPC and Intel minis, both seem mind-numbingly slow until you feed the RAM hole.
IN FACT, my core solo is running iTunes, Limewire, Trasmission, MSN, Skype, Safari, Mail, PowerPoint, Word, another mail application, Remote Desktop, and Parallels right now. I didn't relize how many applications I was running......