IMHO it would be to your advantage in this scenario to go with a reseller plan. The plan you currently have might do the trick, but it really depends upon the size of your websites as to how much transfer you will really need.

I always advise clients to start out small and upgrade as needed, most hosts work well with this procedure. Just make sure that you can upgrade quickly and inexpensively and also that overage amounts are not at arm and leg type prices.