The P4 640, does have the extra 0.2GHz, but also has EM64T (Extended Memory 64 Technology), which offers a degree of 64-bit compatability, whereas the ordinary P4 3.0 is only 32-bit.
The P4 640 also has "Execute Disable Bit", which is a clever little buffer overflow protection against some malicious code and worms etc.
I would choose the 640, but you will need a EM64T compatible motherboard too, and will also need a 64-bit Operating System to take full advantage of 64-bit capability.