Well for starters, P3 systems use standard PC-133 RAM which has gotten dirt cheap. $20-30 per 256 MB module. Most P4 mobos (motherboards) use Rambus memory, which costs three or four times as much, yields no additional performance that I can detect in everyday use, and is IMO a memory format doomed to fail.
There are also some issues about instructions-per-clock-cycle and clock cycles themselves that can cause the P3 to perform better than you might think vs. the P4. I haven't priced out a system in the past couple of months, but I think you could probably save somewhere in the neighborhood of $300-400 for the same level of machine just by going P3 instead of P4. I replaced a P3 with a P4 around six months ago after a lightning strike and it was definitely the wrong move.
RAM makes a huge difference. SPEED OF HARD DRIVE makes a huge difference. These are usually the real performance bottlenecks in a system, not the CPU.
MM