Find a provider that can put you on a cogent uplink and allow you to grow your bandwidth as your grows.
The problem with your situation, is you have a server deal on oversold bandwidth. When you actually go over your quota, your provider is going to rape you. You are probably at your lowest entry price point.
I would think you neeed to be talking mpbs, rther than gb/month for this project.
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Since your web site is a non-profit fun radio web site, the best solution for you would be to choose an unmetered 10mbps offer or something similar which are normally provided on Cogent bandwidth as PSFServers stated.