Someone's clearing out tons of dual processor 1.33 GHz Xserve G4s on eBay for REALLY cheap:

http://shop.ebay.com/?_from=R40&_trksid=p3907.m570.l1313&_nkw=xserve+G4&_sacat=See-All-Categories

While it's a shame to buy one just to pull the processors, it's better that than the scrap heap.