Im starting a webdesign company, the site will be made in flash, and I was wondering how much bandwith I would most likely need

Youll be fine with 10gb minimum.

Depends on how much traffic you will get.. I mean if its a site that is brand new and no one ever really heard of before you should be all set with 3-5gb.. And if you need more just make sure that the host will allow you to upgrade with no fee.