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.