I occasionally play in a campaign that the DM has loosely based on the Dark Tower series. He simply adapts OD&D for that purpose. I've only read the first book in the series but I think he's managed to capture some of the basic elements.
I think Savage Worlds with the supplement Deadland Reloaded, and one or two Sa.Wo. supplements on sci-fi and post-apocalyptic would be the best way to go.
