Shared Hosting, VPS or Dedicated
 

I'm debating if I want to (or need to) move my hosting to another company. At the moment our stats look something like this:

Threads: 25,958, Posts: 368,263, Members: 18,150, Active Members: 5,513

We average around:
60 new sign ups a day; and
500 new post a day.

Our database is currently just under 1GB however we are pushing in excess of 80GB of traffic a month (and we don't host images/attachments so it's a lot of data).

The choices are between shared hosting, VPS and dedicated. Reality is that dedicated is just two expensive for us at the moment, but it's there so I cover all bases. VPS is a strong possibility, shared is where we are now.

What I don't know is where the break point is, how big does a forum need to be before shared hosting just wont cut it anymore? There is obviously a difference between a *good* provider and a poor provider, but lets assume all providers are reasonable for this discussion.

So at what point does it make sense to step up to a VPS service?
At what point does it make sense to move to a 100% dedicated server?

Further, assuming a move to VPS, what sort of specs should be considered? Most of the providers I have been looking at cover available RAM, Storage and bandwidth. I understand the storage and the bandwidth requirements, but when it comes to RAM how much is needed/recommended? Dedicated vs burst-able?

Is there anything else I'm missing or should be considering?