All of them that I've seen have been enormous. Have to mount perpendicular to the motherboard in a slot just like any card.
When Supermicro can sell a multi-P4 machine, then people like me will care that the P4 is up to 3Ghz and has hyperthreading.
Does anyone know if such a setup is in the cards? I'd like to upgrade the database server at some point, but don't want to go dual-Xeon if there's a quad-P4 on the horizon.