All you can change are RAM and the hard drive. Neither will make a speed demon out of your G4.
No amount of upgrading can change the fact that modern web pages are no longer optimized for G-deries processors. We have a number of G-series iMacs here and none are coping well with the Internet.
What you can and can't do depends on your exact iMac G4 variant. A lot happend in 2002-2003. In mid 2003, the logic board was changed to provide USB 2.0 ports and a 67-percent faster system bus. It also gained (unofficially) the ability to handle twice the RAM of the earlier USB 1.1 iMacs.
If yours is a USB 1.1 model, I doubt anything you did to it would show as a perceptible speed increase. If its a USB 2.0 unit, maxing RAM to 2GB may help considering the faster bus. A solid state hard drive might help but it would cost more than any used G4 iMac is worth.
"Legacy" SSD for older Macs are available. See here:
http://eshop.macsales.com/shop/SSD/OWC/Mercury_Extreme_Pro/Legacy_Edition
Also please research the takeapart of the G4 iMac--it is not a trivial matter. If done incorrectly you can fry the computer. Search for "imac g4 takeapart" and see if you are up to the strigent reassembly steps.