I think if you take latest hardware (intel ethernet cards, P4 3000+ you will get plenty of traffic
This depends on how you define plenty.
The other problem that requires consideration with a software based router/bridge is latency. A router has to read a complete packet before forwarding it. With large packets this represents a considerable (and variable) delay. Compare this to cut-through switching (switch decision after 6 bytes) or fragment-free (64 bytes) where you have a very low and predictable latency on each packet.
The number of switch ports that you require is also problematic. Four 4 port ethernet cards in a PC are going to overload the PCI bus completely, but you still have only 16 ports.
Not knocking the ideas here, just trying to introduce some of the other factors.
Regards
Andrew