What is it with people here throwing tons of memory down their Macs? What is it obsession, hope for the miracles - what is it? Granted I have 2.5Gb in my G5 PowerMac - but it's really overkill for everyday tasks. After observing behavior of Mac OS X, you need at most 1Gig and OS actually will be fine with even less. 512 is ok, on my machine the only time I noticed a difference is when having dashboard load while bunch of other staff is loaded. So no, he doesn't need to buy 2.5Gig to notice the difference.
The truth is, that G4 clock for clock is really comparable to G5 performance. And even though there is faster bus, memory etc, unless you doing games, rendering in every day tasks (browsing, documents, slideshow) it will make little difference. So, assuming you went from 1.64Ghz to 2.0Ghz - there is not much difference there.
Keep in mind though that once you start stressing system by having multiple programs running etc, you should see that PowerMac is faster simply because second processor/core comes into play.