Thats exactly what I was thinking - I think if I get a socket 478 motherboard / 256MB AGP and 512MB of Ram - I can do it for around $150. The celeron should be able to handle this right? Or should I spend $40 more and get a P4 2.8Ghz? Thanks.
P.S. How will I know if the Mobo is compatible with the existing processor? Will I need to buy an extra power supply for the the new mobo and AGP? OR will the the one in the HP suffice? Thanks.

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic