You can go as high a C rating as you want, the penalties are higher cost and slightly higher weight.

You can go to Hobby King and get decent deals on batteries. If you want more flight time you have to go to a higher Mah battery, but stick with 3 cell.

Just make sure you dont go too much bigger or it may not fit and it will weigh more. Personally I'd just get another battery or 2 in that size range.