Yeah, I think if you have a P4, then the MOBO and CPU upgrade route is a good one. I'll put it this way:
P4 3GHz era -----> E6600 era -----> E8400 era -----> i7 920 era
You don't need to go socket 1366, or even 45Nm 775 if you don't feel like it (the Q6600 is cheap and great), but the...