Maybe I can help a little. First of all: Your iMac most certainly is a PowerPC. You're correct that it's not 64bit though (ya need a G5 for that).

Your options (that I know of) are:
Yellow Dog
Mandrake 9.1 PPC
Debian PPC
Gentoo PPC

Yellow Dog is essentially a PPC-only version of Red Hat 9. If you use one of the others, you'll need to make sure you download and burn the PPC iso's. Make sure you've got a PPC version, then make sure you burn the iso's correctly (it's different for iso's ya know), and you should be good to go.

Good luck!