Upgrading an old iMac G4

Hello all.

I am looking to upgrade my iMac to the newest OS.
My specifications:

Machine Model: iMac
CPU Type: PowerPC G4 (2.1)
Number Of CPUs: 1
CPU Speed: 800 MHz
L2 Cache (per CPU): 256 KB
Memory: 256 MB
Bus Speed: 100 MHz
Boot ROM Version: 4.5.0f4
Serial Number: QT30500AP19

I know how to open the base and all that - but what kind of memory do I need to put it? Do I need to do anything else other than new memory?

Thanks for your help in advance.

There's no processor upgrade for it, but check the specs and info at http://www.everymac.com/systems/apple/imac/stats/imac_800_fp.html