You'll be driving through Perigord Noir so put megaliths out of your mind and instead think rock art. Paleolithic rock art to be exact.
I can't find a good website with a quick search, but an internet search for Lascaux and Grand Roc should help. http://www.vezerevalley.info/ contains some stuff. Perigord is THE area for it though so for a change of pace thats what I would recommend.
We went to Pech Merle a couple of years ago which is South East of where you are going, but a fantastic example.
If you are desperate for megaliths there are a few standing stones dotted about otherwise it Dolmens, Dolmens, Dolmens...
http://dolmen3.free.fr/lot/