We have a Jetta and a Civic. DD started in a Graco Snugride and moved to a Britax Boulevard. Both seats were in the middle of the backseat and fit fine. The Britax was a bit tight rear-facing, but it worked in the middle. Forward facing is just fine. It would have been harder on the side since we would have had to move one seat up to make room in back, and there is not an overabundance of legroom in that car. Babies R Us will let you take the car seats out to your car to make sure they fit before you buy it. You may need to leave a driver's license, I don't remember.
Good luck and congrats!
I love our Jetta diesel wagon. Great mileage, can haul 4x8 plywood on top, DH gets to pull up by the big-rigs to put fuel in.