If I recall correctly the Hard Drive is a 3.5" format SATA drive.
I had a G5 iMac sometime ago and swapped out the drive for a bigger one myself.
I also boosted the ram, as I recall it uses " dual channel architecture " and I believe you have to install two " matched " memory modules.
For some reason " matched pairs " are expensive but are supposed to be like exact twins when it comes to performance. Having worked in the chip making industry for a while I knew that " matched " pairs were basically just two modules from the same production run.
I went to CompUSA and bought two single modules, BUT, I checked through them all to insure that I had two singles from the same production run.
I believe the RAM Specs are shown in the owners manual, as well should the HDD specs be also.