i think that 3/8 would be too big for 1/2 inch drywall. once it is squeezed in, there won't be too much room left for caulk. we'll see how well it works in practice though

http://www.loghelp.com/backerod.html
also has info on caulking technique
http://www.loghelp.com/info/brodtips.html
and a caulk calculator (caulkulator?

http://www.loghelp.com/caulking.html