Oh, I replaced a 128MB SO-DIMM with a 512MB one for a total of 768MBs of memory in my (company's) PowerBook G4/500. Cost: HK$1,100 (US$142). PowerBook is noticeably faster.

Now if I can only get Mac OS X versions of Adobe Photoshop and Microsoft Office! I actually have company approval to purchase Microsoft Office v. X so I'll hopefully be placing an order on Monday.