Now it really depends...
If your son wants to do the usual teenage stuff such as play games, download music, ogle teenage girls on MySpace, that sort of thing, you really don't need to splash out on a monster PC. All the dealers will be delighted to relieve you of massive amounts of cash, but I'm afraid you do need a LITTLE bit of homework.
Probably the most important question is playing games. Modern games stretch a PC in a way other programmes don't, so does he need a game-playing rig? If so, be prepared to splash out on a better video card and (somewhat) better processor. If not, nearly all modern PCs will do just fine.
If you're not a PC whiz, reliability and security become very important. How will he connect to the internet - and more importantly, what will he be doing there. You MUST consider parental controls, antivirus, antispam, antispyware, a firewall and... the easiest (but not cheapest) is to get a fully integrated package from a company such as McAfee, Norton or similar. Then, you'll need to check up on what he's doing. Believe me - there's stuff out there you do NOT want him to see...
Your specs? Memory - up to 1gig if possible, disk is OK, processor WAY too high (AMD 3200 should be fine, Pentium 4 2.8 likewise), DVD fine.
So sort out the possibility of the gaming situation and look at a few well known manufacturers - might be better to buy it locally, rather than mail order - that way you have a real person to talk to! Alternative is to get your son to make friends with the nerds...
Try
here (cheap, no games, no monitor), or
here (average, play simple games, monitor included), or
here (whoa neddy!!).
Good luck...