hn doesn't have a database capable of master/slave as such...so I think this will be harder if it ever becomes popular enough. I don't think it gets enough traffic it's ever likely to exceed what you can fit in a single box, from what I know.
In the first 99 comments of this page, average comment text size is 231 bytes. Counting all comments in articles on the front page right now, there's 1678 of them, making somewhere around 388kb of comments for the past 12 hours.
So for safety's sake round that to 1mb/day and multiply by site age (5 years).
That gets us 1825mb, projecting forward it's difficult to imagine a time when a single recent SSD on a machine with even average RAM wouldn't be able to handle all of HN's traffic needs. Considering the recent beefy Micron P320h and its 785kIOP/sec, that could serve the entire comment history of Hacker News to the present day once every 2 seconds, assuming it wasn't already occupying a teensy <2gb of RAM.
Even if Arc became a burden, a decent NAS box, gigabit Ethernet, and a few front end servers would probably take the site well into the future. Assuming exponential growth, Hacker News comments would max out a 512GB SSD sometime around 2020, or 2021 assuming gzip bought a final doubling.
Clearly pg should release the dataset and institute a annual round of hn golf where participants compete by recreating hn and trying to get the best performance for a given (changing) deployment target (SSD vs HDD, different RAM & CPU).
