Without knowing any spec on the power supply, motherboard - you may be able to put up to a 2.0ghz P4 - since that was the last p4 that used the 400mhz FSB. Depending on fast of a p4 you want to put in there - OEMs go from $20-$40.
Also, depending on what your son uses it for a memory and video card upgrade would show some extra performance. RDRAM is, unfortunately, not cheap and will run you about $50~ for 256mb and $90~ for 512mb. If you never upgraded your video card (the original is a Geforce TNT2) which is ancient by todays standards, your computer has an AGP slot and theres a lot of cards out there that can up your performance - don't go too high end because you'd most likely need to upgrade you power supply which is just kind of throwing more money away (look for maybe a Geforce 4 ti4600 on ebay for like $30)