Hope you guys get this worked out and you find a host will realistic plans and pricing and who communicates them to you-compared to your needs and use patterns-clearly. If you're running a big, high traffic forum, you need a host that is expecting to deliver those loads & traffic going in...not later as an upgrade.
Big-high traffic forum owners tend to post these type experiences a lot. Often because of the "CPU-memory" clause. Those forums sometimes require more specialized hosting. (and no, we don't do big traffic-high load forum hosting for really big forums...just an observation)
If you keep going with 500 sign-ups a day you may need a dedicated for that one before too long.