RAM Question

I just purchased a slightly used iMacG5-isight, and upgraded it to 500GB Hard disk, (That was FUN!) and took out the 512 RAM stick and replaced it with a Kingston 2GB PC-5300, 667mhz DDR2 stick that says it's backwards compatible to PC-4200 and PC-3200. After installing and uninstalling and reinstalling 4 or 5 tries, I have been completely unable to get the iMac to recognize it. The info I could find online said it could use 2.5 GB max by adding 1 2GB stick in its single RAM slot. Is there any reason why it will recognize a 512 stick, but not this 2GB stick? And I'm a l;ittle confused...I thought to get 128bit memory, I needed 2 matched banks of RAM, but that is impossible in this one.