I am guessing you have an older non-socket 775 as I don't think any of those were 2GHz You could just get the new CPU but then you still stuck with a slow single core and AGP. To be honest if you have the money here is what I suggest. For just 20 bucks more then that old CPU you can get AMD Phenom II X2 550 black edition and then pick up a cheap $50-60 motherboard.
You will see a very large performance boost in everything from gaming to every day things, have PCIe rather then AGP and newer RAM types which in the future as AGP is slowly dying out as far as support goes.
If you don't want to go that route then pick up that one I linked to or if you do have socket 775 you can get a cheap Core 2 Duo but that might get bottle necked by your RAM.
You forgot to mention he needs new ram as well. RDRAM/PC800 is the old "RIMM" memory, and won't carry over to any new build. It is still good advice to dump what he has however. He's got an old system for sure, and will see great gains by getting something newer.
Yes, your right I guess I over looked that thinking it was obvious to most people (I was thinking about CPU's so I wasn't thinking about him might not knowing something like that.)
You will need new ram but DDR2 is very very cheap right now.
Also I thought about it some more and likely your ram is going to do some bottlenecking on even the 3Ghz Pentium 4. To what degree I am not sure, might not be much; it has been a long time since I have dealt with stuff. I remember that used to get hot as hell though.