the OS is the same although there "may" be some specific bits that the iMac used that the G5 Tower doesn't and vice versa.
I believe they both use SATA drives so plug it in and see how you go,
You will not harm it, the worst that will happen is that it will not boot
I got a Dual 2Ghz PPC G5 off of eBay today because my beloved iMac G5 17" (non-iSight, won as a prize opening day at the local Apple store several years ago) main board died after months on the death bed. If I drop one of the old HDDs (I have about three with more or less identical systems on them thanks to CCCloner) in the master slot of the "new" machine will it boot or are there going to be other issues? They both use PC3200 so I can drop lots of old ram in. Also, does the RAM have to go in Parity-style?