Thanks for all your replies - I have a ton of material to read through - I'll let you know around Easter time how I get on!

Not sure whether I need NAT or Masquerade now - here's my setup:

Internet --> [Cable Modem] --> [eth0|Linux box|eth1]-->[Hub]-->LAN

Make sense? Pretty common setup, I think. I need all LAN machines to access internet resources, and I would like most server software on the Linux box - I want the ability to have certain server stuff in the LAN for testing purposes so would need NAT for that (masquerade?).

Thanks - please clear up my confusion.