From my experience, a fast P4 linux box with 2 good NICs can NAT 45Mbps easily. I am NAT/PATing >4,000 desktops with extensive access control lists and no speed issues. This isn't over a 45Mb T3--this is over 100 Mb Ethernet.
--Patrick Darden --ARMC, Internetworking Manager
A second CPU or core will help tremendously. We used to use single-CPU boxes for this and we noticed that traffic sometimes stalls when the machine has to do some task other than NATting, such as expiring idle flows. Having a second CPU or core will help keep latency much more uniform. We have a few dual 3.2Ghz Xeon boxes (not the ones based on Core, the older ones) that NAT/FW across two GE interfaces. They do quite well up to about 300Mb/s, then we start to see issues. We believe the issues are due to overloading the NB-SB link. A more modern mobo probably wouldn't have this problem. DS