upgrade 2.0ghz p4m...what's best direction?

Running an Intel 2.0Ghz P4-M, 1Gb RAM, and am thinking about a CPU upgrade. I've been told that I can go as far as a 2.6Ghz P4-M on the Intel side but can't go to a straight P4 even if it's Socket478. Why is this and what other cpu might I be able to go to that would give me the biggest bang for my buck? This is a Dell Laptop running Linux and used for both work and home with Vmware running and other home grown GUI applications.

Thanks

Kevin

you can't switch from a p4-m to a p-m becuase of different sockets, one pin is located different. You won't gain a lot of performance over your older 2.0, and it will cost you alot.