At both FNAC and Virgin Megastore, you can scan the bar code of any CD to listen to samples of it. So you might just take a look at the top ten (or whatever) of the French music section and see what appeals to you.
Almost all French music is terrible. MC Solar is bloody awful. Daft Punk is about the only decent French band I know of, although a few people in Stereolab are French. If you like inane jazz, they do that quite well, or serious classical, not bad. But popular music in French, it doesn't really work. Get an iPod & a gizmo to let you play decent music through the car stereo. French radio is, well, quite unique, and made worse by my understanding of the presenters wittering away to get the French language content up the air. They have laws about the amount of non-French words used on the air.
The country is lovely. Forget music in France

Some oldies besides the aforementioned Piaf and Brassens: Charles Trenet, Juliette Greco, Yves Montand, Charles Aznavour.
And even older: Django Reinhart and Stephane Grapelli (jazz guitar and violin, 30s and 40s.)

And we mustn't forget Plastique Bertrand. No link provided. I will not taint my 'puter