www.meshcomputers.com
I have one from there cost me £550 but thats with a ati x700 pci-e card.
You could configure the same system without the card for about £420 perhaps, which would give you a PCI-E mainboard a Athlon 64 3000 newcastle core processor 512mb DDR ram, and a 80gb SATA HD, with keyboard and mouse.
However for a monitor you would be stuffed although I have a 17inch CRT I could let go for about £20.