WHY or what do you need to do or want?
Mac Pro may be overkill coming from G5 1.6 or 1.8DP or 2.0DP.
You say upgrade but what you mean is replace with something new.
To get Apple security updates, support iTunes 10.7+, iCloud and all the iDevic-ification of Mac OS.
Mac Pro 2010 Ref'd Special 2.8 4-core $1819 Apple Store section. Or new iMac $1200.
What I meant to replace internal components not to upgrade to pro. So can use current version of all apps as you stated e.g ITunes 10.7.
Last night bought my son new iPod but he can not use it due to older I tune and new version require updated os and processor is old for that,you got the issue.
My needs are not extreme,just wondering will be cheaper to go this route or not.
This is not the PowerMac / G5 forums. So sort of assume some things....
you can't upgrade processor; you can't go beyond 10.6.8 I think (or was it 10.5.8?)
Buy a Mini or Apple Specials for iMac are great.
So if you want specific help you have to be more specific also.
Macs use to allow G4 to G4 upgrade processors, not G5. And that won't change anything.
You can't upgrade a Power Mac beyond 10.5.8.
The latest iTunes for PPC is 10.6.3. iOS device support needed in 10.7 is reported to be fine in 10.6.3.
That may suffice, for now.
Note that the iTunes update requires OS 10.5.8.
Beyond this, a new(er) machine is all that you can do.