I'm unsure ad space could cover the loss bandwidth. You may want to think about going with a VPS or Dedicated Server. Sites grow with time and yours seems to have hit its toll for shared hosting.
My 2 cents
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Surly you need a dedi. It's quite natural your site grows up. You know you may have a good dedi from ahosting.biz. This company is quite reliable and their support is fast and responsive.
I can offer you some shared hosting in exchange for ad space. How much space do you need. Allthough i would reccomend you purchase a VPS or a dedicatated server from Greenlush.com
Awaisuk – consider having a check at www.eukhost.com for your needs of perfect hosting. IMO Dedicated is the way to go for you but initially you can also opt for the Semi-dedicated plan.