It depends. It can have a huge impact, or only a slight. Depends a lot on what you have currently. The L2 Cache amount, whether it's HT compatible or not...etc.
I went from a 2200+ to a 3200+ on AMD and saw a significant boost.
thats. do you happen to know whats faster a 3200 amd or a 3 ghz p4? just incase i feel like spending the money on a new motherboard and chip for the possible boost in speed if its the case.