You can't run any P4 CPU in dual-CPU mode (this is called SMP). If you go with Intel, you'd need a Xeon CPU. AMD does this with Athlon XP's (special dual ones and regular ones when using modded BIOS') and the Opteron.
Where you benefit from a dual CPU config? When using applications that can handle SMP (apps that don't know about SMP often run slower on SMP systems than on equivalent single CPU systems), running on a SMP cabable OS (that's either WinNT/2k/XP or Linux for most ppl).
What do you mainly use your system for?