I would say linux in some fashon would be the best bet. You can log just about anything, and its pretty light as far as the hardware it needs. In theroy if its hooked up and configed right i dont think it would slow your network but thats merely mho. Linux has all sorts of goodies for doing exactily what you are talking about
NAT
IPTABLES
TRIPWIRE
PORTSENTRY
TIGER
ETHEREAL
as well as many other goodies ...
hope this helps