Consider about 50k bandwidth per player. for 5-player servers. This gives room for other misc overhead.
Machine-wise, I ran servers for LANs, and it worked out that I could hold 4 1v1 games on a p3-800...so any machine ~2ghz can handle anything you throw at it. D3 is graphic intensive, but the server doesn't take all that much. You could hold 4, 4-player games with any decent machine nowadays. Make sure you have a gig of ram. D3 likes ram.
There are four boxes to use in the defense of liberty: soap, ballot, jury, ammo. Use in that order.