IMO, these days Gbit network is a must. If you want to install a lot of machines, it would be perfect if your server has that much RAM, that all packages for the clients will be cached in RAM if your install server is also the package mirror or proxy. If you install around 2GB of packages, this amount of RAM would be perfect for your install server. In at the end of fai.log you will see how much network traffic was received/transmitted. It looks like this:
eth0 rx_bytes 2113.26 Mbytes eth0 tx_bytes 6.15 Mbytes -- regards Thomas