Depends on what he wants to do with his system.
Where is he running into performace problems? I don't know what else he has in his rig, but I can't think of a single desktop program that won't run well on a 1.8GHz proc. If you do digital video editing or play hardcore flight simulators, you'll save some time or pick up a few FPS by going to a 2.4-2.6GHz system. But that's it.
If he's asking if its worth upgrading, that tells me he's not having a specific problem but fears he might be missing out on something. I think that's a lousy reason to upgrade, but it his money.
Might I suggest, however, that $500 spent on a decent 21" monitor or a top of the line speaker setup will be an improvement he'll notice every time he uses his computer. Same for a good set of game controllers, or a wicked chair like a Herman Miller Aeron.
Just a thought...
BHD