Change the order to a P3V4X
the P3 is Slot1, has only ATA66 and 4DIMMs
the CU is S370, has ATA66 and ATA100, and only 3DIMMs
Other than that the boards are the same.
ATA100 won't make much difference and with a Slot1 motherboard you can use Socket370 (with converter) or Slot1 CPUs, and you get 1 more DIMM.
If you get the CUV4X you can't use Slot1 (no converter for that way) and you have 1 less DIMM. Oh no..no ATA100...makes no difference at all..the only way it might matter is if you have 2 IBM 75GXPs on a single channel. And even then you might manage to peak at 70MB/s instead of 66..whoopie..
The P3V4X is a better choice for all-round flexilibity.