Personally, I'd say it depends on what you currently have that determines your need to upgrade. Right now, I've got a P4 2.4C a GB of DDR400 and an AGP 6800GT. It runs every game I throw at it at 1280x102 with details maxed out. Given this, I cannot justify spending a grand to upgrade the CPU and motherboard again, for a minimal speed boost.
My next upgrade is likely going to be in 05, to some sort of dual core CPU, more memory, and a PCIe videocard. Probably sell almost everything in my current system though. 🙂