[tor-relays] Cheapest HW to get 20Mbit?

Hi Since a few months back I have two RPi running Tor relays and delivering about 1Mbit each of peak throughput. Now my ISP has increased my max upload speed and I am looking to contribute more, somewhere in the 20Mbit range or possibly more. But what hardware do I need? Has anyone written down a list of minimum requirements to get a certain throughout? I have tried to google, but failed. Sincerely Daniel Armyr.