I think he use a very expensive own server.
HF is on 3 servers with one dedicated strictly for the database.
But that's not because of the database size, that's because of the massive concurrent connections right? I don't want the OP to get confused and think that big DBs require their own server unless they are getting a lot of active connections.
You can't get a massive database without massive traffic. They are not mutually exclusive.
Not really my point, but ok.
Well technically I know of one site larger than HF that is running on one server.
http://forum.yugiohcardmaker.net/index.php
But he doesn't get attacked probably nearly as much as I do and I know HF is more active. I'll pass his post numbers in just 2-3 months.
My database is right now doing 360 queries per second on average. I'll probably need a slave MySQL server soon to seperate the reads and writes.
Hostgator is my suggestion.
Do NOT use anything like Sitefrost! Slicehost & Linode are quite good.