Motherboard/Processor update

Your CPU socket is PPGA478, P4 is the highest CPU this socket will support.

Even with LGA775 CPU socket, unless you had a motherboard that would support quad core, there would be little reason to upgrade over a complete system replacement.

In answer to your question to "Plug and Play", yes and no. Yes CPU's are basically plug and play, and No the motherboard must support the CPU first.

For 400-500 dollars you can buy parts that will make that P4 system look like a pocket calculator. I went through the same folly a few years back, trying to upgrade a P4/775 system - all money was wasted, except the $100 Corsair case (still using it). Recent hardware is priced very reasonably for the performance you get. Bus, RAM, and CPU speeds have all increased by large factors, and competition among vendors is stiff. Even for machines that only do office duty, the difference is incredible. Do a little research and you'll see how easy it is now to build a fast, economical PC.