I'm looking into switching to a new server, but I'm not certain exactly how much bandwidth I need. My network has been pretty low profile for a while and currently doesn't get many visitors during an extended development and testing phase.
Anyway, I was wondering if there was a guide or if anyone had tips on how to figure out exactly how much I need? I mean I know how much I've been using lately, but I'm planning to open a whole lot of stuff at once, so I want to make sure I don't underestimate and screw myself over with too little bandwidth, but neither do I want to pay for too much.

Thanks for any help :)

Your best bet would be to find a host that offers at least five gigs of transfer (doubt you'll need that much just starting out, but that's a good minimum plan) and make sure that they have plans that will allow you to grow. Email the host and see how they handle upgrading your account and what steps are involved as you grow.

For some, it could be something as simple as just giving you more space and transfer, for others it may be moving yuor site to a different server. Others may not be able to do it at all.

find a host where you can easily upgrade your account if needed and pay for something cheap at first

You will find out after one month using it and most host will give you option to upgrade without hassle.

Agreed. As long as the host offers larger plans you should not have a problem.