How do I buy a SIM card for my cellphone in Paris? Also, is there anything I should keep an eye out? (things like fees, cellphone-compatibility in europe...) and do they have pre-paid plans (in Brazil we have those, I don't know how it's called in other parts of the world, but its simply a plan wich you pay before using, and once your credit is over you cannot make any more calls before recharging the SIM card).

If you know any, please post some buying places near the Arc de triomphe.