Insulation, Radiant and Vapor Barriers - wall insulation

i am planning a project and i'm going to remove the drywall in one room and reinsulate the wall. going to put in r-15 fiberglass insulation into the wall cavity. wanted to know if i could install foam board over the fiberglass between the studs and new drywall. cornings website show this material being install on the outside of the wall under the siding but i'm not planning to replace siding yet. this would increase my r rating to 20....any suggestions will be appreciated John in MD

http://www.eere.energy.gov/consumerinfo/factsheets/bd4.html

This site shows and explains the application of vapor barriers. The R-15 has a vapor barrier and the foam board has a low Perm Rating. If you apply the foam board over the R-15 you increase the probability of a moisture problem.

would it be better to fur out the wall 2 inches to increase depth to 6 inches

R-15 means 1/15th (.067) of a BTU travels through this material every square foot an hour. R-19 means 1/19th (.053) of a BTU. The difference in savings is 6.7% - 5.3% = 1.4%. This exemplifies the "Law of Diminishing Returns". Of course installing the R-19 is better than the R-15 but if the R-19 requires you to build out the framing to accomodate the R-19, it is not considered practical. If the wall does not have any features, like windows and doors, framing out is fairly easy. I can assure you most building departments feel the same way.