That means you likely have a P3. To upgrade to a P4, you need a motherboard or hold the P4, new power supply since it's quite hungry and the RAM changes too. With all the heat, a new case is fitted.
If you were to stick with the P3, it's unlikely any real gain of moving to the 1.2GHz would be worth it even if your old machine could support it.
Bob
I want to upgrade my 1ghz processor. Any recommendations? Soemthing around $100, maybe a Pentium 4, between 2 and 3 ghz. Also need to know how difficult/simple this is? I've done quite a few upgrades that were all easy plug and play. Is this the same?

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic