If you added a 3 GHz processor with at least a 1 MB cache that would speed things up. A little more RAM would help too. If you want to do a major upgrade, you could get a new motherboard that supports PCI Express, change the graphics card, and go to a dual core processor. Ultimately it comes down to what the computer is used for and if the cost could be justified by upgrading it.
Well, it looks like the overall low spots are the speed of the RAM (it's PC2700 333MHz, while PC3200 400MHz and even 500MHz are standard). And the video card is probably not the best either.
Does he want it for gaming, or for video editing, etc? The most powerful card you could probably get that fits an AGP slot are the ATi X850s or the nVidia 6800s. I still have AGP, i'm planning on upgrading to PCI-express later.
I'm assuming that the computer only takes a max of PC2700 ram (Or i'd suggest a gig or so of PC3200). Othere than that, really the video card needs upgraded (Although a 5500 is certainly better than the one he had!). The CPU seems to be a farily new one.
System builders usually lock out the bios so you can't over clock but you should check whether you can or not... that may bring new life into your system.
Also, the best AGP card available right now is the 7800GS, pricey though. Add some more ram as well, keep the old ram so the new stuff can be PC2700 as well because the motherboard will default all the ram to the speed of the slowest.
If he can handle using that with a few minor upgrades for another 6-8 months, build a Vista machine early next year with a core2duo (maybe even K8L?).
If you put a video card in with VIVO such as any of the ATI all-in-wonders, once you build a new one, say for VISTA and DX10 gamming, you can use this old Sony as a Tivo, or any number of other things.