I've got a iMac G5 20" 2.1ghz (iSight). It has 1.5gb of ram with 512mb built in. If you could, link me to an affordable but good stick on newegg or something.
Your computer will take up to 2.5gb of ram. Everymac.com is a good resource for any type of questions about your system configuration. LINK
Your looking for 533 MHz PC2-4200 DDR2 SDRAM. I'll let you look that up at the vendor of your choice for prices. Make sure to get Apple compatible ram and don't listen to anyone who tells you there's no difference. Not sure where your at, but I usually get this kind of stuff from my local Fry's store. You could also order from them online if you wanted.