I'm one for upgrading a few components at a time but i've built 3 complete systems from scratch. Sometimes it's hard to upgrade one component at a time when you need a new Mobo & Processor. My next upgrade will be a 2.53 P4 and new Mobo to go with it. I'll pass my Athlon XP to my secondary machine.
All that upgrading cost somewhere in between 1500 - 1800.