Socket 423 is the origial socket for the P4, only supporting the Wilmette core (.18 micron die), It will end with the P4 @ 2.0 Ghz. there is no ability to upgrade to Northwood (.13 micron die).
Socket 478 is the current standard, you can get P4 Wilmette or Northwood versions in socket 478.
Northwood is the transition to a smaller die (the size of the chip and its internal transistors) and increased L2 cache. there are a few other changes not release to public yet. Jan 7 2002 is the official launch of the Northwood.