OK, Everyone calm down and take a few deep breaths. .... I believe I can help sort this out, and being an Idiot, I could be wrong.
I think what Benzilla is asking is; < Can he use the the HDD('s) from his PPC G 5, with OS and APP's in an INTEL PRO MAC (desktop) with success ?, or, In other words, will it work just transplanted with OS & APP's going from a PPC to an INTEL MAC PRO ?
The Answer I believe is; NO !! ... the OS's are completely different for INTEL MAC's, and the APP's won't work either, even if you changed the OS, as they are also different PPC to INTEL, ( a few may work under the same name, but a different download for Intel, so Basically NO !)
The last part of the question; will your HDD work in an INTEL machine ? .... from what I've read here (remember I'm an IDIOT) ..... if their the same type, I.E.; SATA (1, 2, 3 (?)) ... If wiped clean and reformatted, EVEN the FORMATS ARE DIFFERENT !!! ..... Then you can load an INTEL OS on it, then add APP's you want, in INTEL format.
OP. SYS's go up to 10.5.8 for PPC's ONLY, ..... NEWER, OS 10.6.x and 10,7.x are INTEL ONLY.
OTHER FOLKS WITH MORE REAL SMARTS WILL CORRECT ME IF I AM WRONG ABOUT HDD TYPES IN ABOVE PARAGRAPHS.
MONEY IS A REAL PROBLEM FOR ALL OF US. ... So if your loaded with it, when you buy your INTEL MAC PRO, you can equip it with SSD (Solid State Drives) as they don't wear out and are faster than greased lightning and hold TERRA BYTES of stuff.
All of this newer stuff is out of my league and over my head, I am still pounding 2 rocks together here. And often hitting my thumbs !!!
Hope this helps with your question, and every one is now relaxed, oooomh, ooooomh, ooooomh ......