I have a couple Mac Pro’s at work (well, 3) that sit behind my desk doing render jobs, and running BOINC. They’re Mac Pro 1,1’s, so they date back to late 2006, but I’ve upgraded them with dual quad core processors, so they do okay. I’ve been keeping an eye out for ATI 5870’s to put in them for some GPU grunt, but they generally sell on eBay for
Make sure to throw the aluminum (aluminium) in the recycle bin. =)