You want speed. For what? Gaming, video editing, 3d graphics generation or number crunching? The choice of processor brand changes depending on what you want to do with it.
P4 is a good overclocker. Make sure you get a MB that supports it properly though. Pentiums must be overclocked on the front side bus (instead of multiplier). So you need a MB that can set your FSB, RAM, AGP clocks independently. The 875 chipset is the fastest so far. ASUS makes good MB that are overclocking friendly. Make sure it supports PC3200 (400 mhz) Dual DDR memory.
Dual processor systems only work if you use an operating system and programs that are designed for dual processor. That means server applications, not desktop stuff. The P4 chip has built in capability to do something like dual processor (it's called hyperthreading). The chip processes multiple threads simultaneously.
The P4 extreme is the fastest chip out there (or was a few weeks ago) AMD just had a new 32 Bit CPU release and it may have pulled ahead.
Whichever way you go a good fan is crucial to your abiity to overclock. Don't get cheap on a CPU cooler. Stock coolers will not cut it for overclocking.
No matter how fast your CPU and MB are if you put in a crappy video card the system will drag. Expect to spend as much for the vid card as for the MB & CPU. Same comment applies. The best Video card choice depends on what you want to do with the system.
