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.
If you are likely to upgrade the processor at some stage then the 478 is your only option. Also, the general consensus is that the Northwood core will be substantially superior to the Willamette core (probably worth waiting 7 days to find out!).
<font color=blue> Smoke me a Chip'er ... I'll be back in the Morgan </font color=blue>
Wait for the 1.6 and 1.8 northwood cpus. They may be in your price range. Also look for more reviews on the 333ddram boards. Atacom has the asus p4s333 board with raid and the sis 645 chipset for $129, but no reviews yet on this board that I can find.