Any low end current pc with enough memory should be adequate. Fro example get cheap celeron with a low end motherboard and 1GB of RAM and you will be ok. However as I said before ZS doesn’t load balance the incoming traffic to multiple servers.
i guess it depends on how much clients you will be expecting… and the kind of traffic they will be having… 100Mbit/s means that your CPU must be able to handle 200Mhz of input/output(without the router delays, SPI firewalls, and other stuff…)
So if you are thinking about having a Pentium IV 2.4Ghz with 256Mb @ 266Mhz i think that will be more than enough.