I'm sure it depends on how the application works, but I have a g5 1 x 1.8 and a quad 2.66, and the quad blows the G5 away in some apps, but in others you don't see much difference.
The big differences are things like video ripping - handbrake for example, which I use to rip my DVDs to mp4 to put on my Ipod, absolutely flies on the quad. It runs roughly 10 x the speed of the G5.
At the moment the quad is a bit short of RAM too, it only has the standard 1gb. I don't know if more RAM will speed things up more.
Hard disk intensive things don't make much difference - I did some heavy rearranging of files last weekend, moving blocks of 50 - 100gb or so around the disks, and that wasn't noticeably faster than it would be on the G5. No surprise though, clearly that's limited by the speed of the disks, it doesn't need much processor power.
But the answer is really that some things are much faster, some are about the same, but I haven't found anything slower.
On the other hand, unfortunately I have to say that in my experience it's not the 23x or 50x you were looking for. Maybe when apps are written specifically to use the multiple cores things will perk up, but the leap isn't as big as that.
I'm very pleased with it, it was well worth buying as far as I'm concerned, but mines just my home PC. I don't do any 'work' on it at all, so I may not be as critical as others.