although your insulation level sounds low, if you are experiencing extreme heatloss (other symptomns other than high bills are unusual snow melting on the roof, ice build up on the eaves, moisture in the attic), increasing the insulation level won't solve the problem. This is not to say that adding insulation isn't a good idea, but there are things that need to be done first. Te first thing to do is to find areas of air leakage into the attic. You may need to talk to a professional in troubleshooting such problems to do this (Talk with your local utility company. the can probably lead you in the right direction).
When it comes to adding more insulation, your far from needing to worry about weight on the drywall, so don't be afraid to blow more on top. I would usually blow cellulose, in part becuase it help to seal leakage that fiberglass can't (It also has a higher R per inch than fiberglass blown or batt).