In researching the Fall of Rome and early medieval Europe I have come across some interesting websites. Regnum Francorum Online is a website that presents in cartographic form information on the Kingdom of Francia, the predecessor of France ruled over by the Merovingians and Carolingians. If you want to follow the routes of Charlemagne or Carloman or find out how much land was owned by the crown or various monasteries and church establishments, this is a very handy website.
Another excellent set of maps, with ancient place-names, can be found on the Digital Atlas of Roman and Medieval Civilization. And, if you want to check out original documents (mostly in Latin) about the early history of the Germanic peoples, you can check out the digital version of the Monumenta Germaniae Historica. I am reading Gregory of Tours at the moment, the Penguin Classics edition, and this definitely helps in figuring out what the ancient names of these cities were. I much prefer Arvernum to Clermont-Ferrand, at least when I am considering the pre-Medieval city.