That doesn't make any sense. You're assuming that they have 10 times the content, need 10 times the processing power, and that they derive no advantages to caching (from 48 GB RAM to 480 GB RAM).
The scenario is 10x the traffic, not 10x the content. And given that Digg is an update-heavy site for all the voting (which affects how pages are rendered for your friends) assuming that caching is a magic wand here seems bogus.
That is not how capacity planning works at all.