Pentium 4 Motherboards
Socket 423
Socket 423 was the first Pentium 4 socket. It was rather short-lived, but still went through a few changes in it's life. Most Socket 423 motherboards used RDRAM instead of the more common DDR or SDRAM.
Today: Theres not really a reason to build such a rig these days. The 2.0GHz Pentium 4 is about as fast as a Tualatin 1.4GHz while consuming a lot more power.
Socket 478
Today: There are four major cores available for this socket: Willamette, Northwood, Prescott and Gallatin. The most interesting are perhaps Northwood and Gallatin. Gallatin is the Extreme Edition chip, a rebranded Xeon core with L3 cache.
Socket 775
Otherwise known as LGA 775. Prescott and Cedar Mill Pentium 4 CPUs, and of course Core 2, used this socket. Early LGA 775 boards do not support Core 2 although later boards often support all LGA 775 CPUs.
Today: Not very interesting for old games because these boards typically do not have AGP and may not support Windows 9x.
