Well, I just did the big huge massive upgrade myself, from an old PowerMac 7500 running 10.0.3 with more bypasses than I care to admit to a new G5 dual 2Ghz with 1 GB RAM running Panther.
I can only say WOW! It is much faster than the Windows box (XP Pro, 2.4 Ghz Pentium IV, 1 GB RAM) performing nearly every task--- woooot
Now of course, most of my software programs are Classic apps and do not run as well in Panther (don't buy the Apple line saying it does--Classic apps run poorly or at least mine do), so I have to take a close look at what to upgrade from my software selection.
I am not disappointed at all!