I have a little Subaru Impreza station wagon, and the bike rides inside the wagon. If I have more than one bike to take somewhere, I put on a Thule trunk-rack.
I see a lot of sedans with roof-racks and hitch mounted racks. You might want to bring along a little step-stool for using the roof-rack. (Trek420 has a roof-rack on her Oldsmobile Alero, and this great little stepstool that makes putting the bikes up pretty easy!)
If I were likely to be taking more than one bike around all the time, I'd consider a hitch mounted rack. (mainly because I *know* I'd forget the bikes were on the roof and I'd cause some sort of bike damage.)
"If Americans want to live the American Dream, they should go to Denmark." - Richard Wilkinson