Hi jack,

Anything over the stock HD in these machines will provide a boost - I'd look into the most modern 5400RPM drive you can get, like an 80 or 120GB part.  Carbon Copy Cloner for PPC is a good choice and freeware:

http://macpowerpc.com/2012/04/02/carbon-copy-cloner-3-4-4/

Ideally reinstall the OS from scratch for better performance!

JB