will an iBook G4 support 1Gb memory upgrade?

Yes.

(64526)

Yes, to confirm I own a M9426LL/A with 1.25GB (256MB soldered to motherboard + 1GB upgrade).

Be careful buying memory, it must be Apple-compatible. This means you will likely need to find *low-density* RAM. Much of the cheap stuff on eBay is high-density and won't work.

Hi, and welcome to Apple Support Communities.

iBook memory (RAM) guide

http://support.apple.com/kb/HT2206?viewlocale=en_US

i upgraded my ibook memory to 1gb. a good place to buy ram from - and its good, not crappy - is from dabs.com