Generally there aren't many bad configurations.
This one isn't either. Just a few things.

1. the p4 with 400mhz fsb is the old version. he's trying to dump
it on you. the new one's have 533 fsb, there called "Northwood"

2. the old p4 in combination with DDR memory perform rather
poor in comparison with Athlon cpu's.
p4's up to the latest chipsets from Intel run best with Rambus RIMM's.
that's what these chipsets where designed for.

3. lets talk about chipsets. right now they're about the most important part of the pc.
There the device's that get all the other stuff work together. Memory, CPU, PCI bus, AGP bus......
With a p4 cpu you shouldn't get anything else than an intel chipset.
they are the: i840, i845, i850 and so on
i guess via is also getting there, make a big circle around the others.

4. ask which brand the motherboard is. it should be an a brand like:
Tyan (too expensive), Epox, MSI, Asus, Abit, AOpen,
about in that order.

5. get as much memory as you can afford. 512mb is no luxury.

In general i would go for an Athlon 2.8Ghz / nForce 2 combo, with ecc DDR's,
that seems to be the biggest bang for the buck combo at this moment.

Do some research on the net. a good source for hardware info still is:
www.tomshardware.com
he's been around for ages.

good luck shopping