Diagnosing a slow network

[heidi at midnighthax.com]

I'm trying to work out why copying a file across my network is so slow.
There are four Linux boxes connected to a 100Mb switch, and copying a 300Mb
from one system to another runs at 500K/second. The systems aren't busy, so
it could be a network card problem, a hard disk being slow, or - what? I'm
using scp to do the copy, so there will be some overhead with encryption,
but 500K/sec is way too slow. Any ideas on how to troubleshoot this?

H