sure you can change it yourself, just get a processor that's the same socket and make sure the speed is compatible with your mobo. some other people will help you on the installation itself, i know the jist, but i have never done it before.
Nah its easy to change out a CPU - assuming its compatable. Given that your going to need a new P4 anyway its already a pricey CPU without adding on payment for some bloke to do 2mins worth of work.