Hey I posted the same thing down below, "To Mini or Not".........
I dont know of any /new/ custom built HP, DELL, GATEWAY or any major
PC that you can buy for 400 or less; unless you already have a
monitor.
However you can certainly build a good PC for 400 or less with room
to upgrade in the future.
Try newegg.com, I have already pieced together several configurations
for a new computer with their site alone and they have come in under or
just above 400.
Also look to pricegraber.com for good deals, be sure to check the seller rating.
Here are just some of the prices out there,
Barebones-168.00(Case/Motherboard+PS) FREE SHIPING
DDR333 RAM-42.00(256Mb Crucial) FREE SHIPING
INTEL Celeron-55.00(1.7Ghz 400FSB) FREE SHIPING
HD-52.00(40Gb ATA100) FREE SHIPING
You can go cheaper, for example by selecting a cheaper Barebones kit, or
less RAM or 1Ghz CPU all with the intention of upgrading later.
I hope this helps somewhat.