SBC has the best prices. They also give upgrades more often. I was able to negotiate to a car equipped with a GPS twice while in Florence, for no extra cost.

http://www.autoeuropa.it/en/Prenotaz...lin=en?tipo=10

I also rent from Thrifty, though they have the habit of giving cars with automatic transmissions to Americans; and these tend to be very small, like the Daihatsu Cuore.

Make sure the "residency" item says U.S. on company's web sites. Prices are much lower than the European equivalent (i.e. UK).

In regards to gas, the stations off the freeways accept credit cards. Like other mentioned above, stations in town or off the beaten path accept only cash.