In Virginia's climate, attics should be insulated to at least the "R38" level, however most houses have less than they need. With either fiberglass bats or a layer of loose fill cellulose, there should be at least 12 inches of insulation. A good rule of thumb is that you should be able to see only the top of the attic joists if you have fiberglass, and not see them at all if you have cellulose.
Adding loose cellulose insulation is easy and it will fill in any gaps you have in the existing insulation. Machines to "blow it in" to your attic are available at many home improvement centers, and are often free to rent with a minimum purchase. And it's an environmentally-friendly choice since cellulose is made up of ground-up newsprint.
R-value is a measure of the ability of a material to resist heat flow. The higher the R-value, the greater the insulating value. Depending on the material, insulation has an R-value of 3 to 4 per inch. That means that full and uniform coverage is critical. If you use "R-38" for example, but the insulation does not cover the attic joists or has small gaps or holes (equivalent to 3%), it would actually lose nearly half its value--and function just as R-20 insulation. For more information on R-values, visit the US Department of Energy "Energy Saver" website.