Probably surprisingly little. Also, the publicity HN brings is most likely orders of magnitude more valuable and that makes it much less important to try and op

Probably surprisingly little. Also, the publicity HN brings is most likely orders of magnitude more valuable and that makes it much less important to try and optimise the service. The pages are very light and the algorithms seem to be designed to be quite light, too. Now, I know neither Arc nor the traffic stats, but I would not be surprised if one or two machines were able to serve entire traffic.