The iBook shipped in only one model, with 128MB of RAM. The PowerBook G4 started at 128MB of RAM and topped out at 256MB of RAM. The Power Mac G4 started at 128MB of RAM and topped out at 512MB of RAM.
The smallest amount of memory you can get on a Mac nowadays is 2GB, and you would've had to buy an entirely new computer at least once to get Lion running in the intervening time.
(Whereas, if you didn't buy a new computer since Windows XP came out, you could run the 32-bit version just fine with 1GB of RAM.)
Methinks you doth protest too much about expensive upgrades.