It really depends. You can do it to a certain degree of cost-cutting, but it only goes so far. I know one forum that gets around 90,000 members and even more guests. It's not the storage that is problematic, it's the bandwidth - he only uses around 10gb for the actual forum (which seems big for a forum, but hardware wise it's not very big), but the bandwidth usage is insane. That puts a lot of strain on the server (which you definately need at that size), and parts then need replacing more often etc. That's what i hear, anyway
Personally, i wouldn't try and make your forum too big. Be satisfied with what you can get - start small, and if you get bigger, that's just great. While i like the idea of having a big big forum and site, i don't like the idea of paying for a server and replacement parts. Remember, people don't buy a huge house if they've only got 2 chairs and a table