NightShade737, we have 20 of those VIA ITX machine in operation as small web and file servers. They do work fairly well, and would be a improvement over your 400 machine. The only consideration is shoutcase, but at only a couple users it should handle it.
We use the 800 proc, single 512 unbuffered DDR ram, and a current EIDE HDD and they work surprizing well. They do make a 1 gig machine now i believe.
The entire lot draws about 5 amps and has very very very little noise. The PSU units for those machines are external and fanless. Swap the CPU HSF out for a larger chunk of metal and you would have something that only generates drive noise. But as it sits, they are super quiet.
Otherwise, I would go with a standard p4, celeron, or for amd we use a HSF based off a slightly larger chunk of aluminum, 80mm adapter, and slient style case fan. These would be fairly quiet given you use a large case with no case fans.
AFAIK, these are your only options.