Would arpwatch provide any extra information? Ntop sounds like a very interesting idea, I wasn't aware. I suppose you mean you can use a wireless card on you Pi, so the clients can connect through it, and then use ntop to measure the usage... Right?
It wouldn't provide extra information but it would be cleaner way to follow the network compared to reading dhcpd leases. You could have RPI (or other comp) working as the wifi AP or you have RPI between the WAN router and the actual Switch/Access point. This way RPIs slow IO wouldn't slow down your intranet.