Hi Peacebomb,
Once the macs are on a network, you could:
1) Enable telnet or ssh on one machine so then you could access it through the telnet/ssh applications - only use ssh if you are open to the internet
2) 'talk' to the other machine - a unix protocol that happens in the primary console. The talk daemon needs to be activated.
3) If you want to run applications on one machine and see it on the other then you can install XDarwin and use X-windows applciations
4) For ftp, setup the ftp daemon on the server and ftp files back and forth.
Your macs will have to be assigned IP addresses. I'm not a network person and not sure how best to do this but maybe you could setup a DHCP server on one. You could also just configure the network and provide a static IP. I use 10.0.0.1 and 10.0.0.2 (this address range is meant for this purpose I think).
Cheers,
Brooke