Intel will be to 3 Ghz at around years end. Sometime in 2003 they plan on releasing a .09 micron P4 (Prescott). I do not know what they plan to do as far as socket size. Maybe someone else can answer that... but I would imagine the voltage will change. Might depend on a new BIOS from the manufacturer.
By the time you get a new P4, you might also wantt to take advantage of new advances in memory technology. I only do a major upgrade every couple of years, so I pretty much always plan on a MoBo and Memory too.