Do you want a fast or faster processor, or do you want to get more work done? The P4 is faster but the Xeon can accomplish more with four cores working vs. the P4's one. Also, since the Xeon's running slower it's running cooler, adding to the lifespan of the CPU and therefore the system; since it runs cooler, it needs less-intensive cooling so it uses somewhat less power as well.

Some people might say that the Xeon is designed for server use so it's not as capable as a Core2Duo, but I think that holds as much water as the Opteron vs. Athlon X2 argument. The Xeon has 6MB L2 cache capacity vs. the P4's 1MB, so a lot more work is done by the CPU rather than using the RAM. The drawback - if there is one - is that the T5400 uses ECC (error-correcting) RAM which I understand is slower but more desirable for servers and workstations where speed isn't the primary concern.

I guess it all boils down to what you plan to use the system for, whether it's for comfort or speed. Now, if that Xeon would work in a Socket 775 that might be interesting.