Andrew-

The correct tire size is the one listed on the gas door. The difference between the 205/55s and the 205/50s is approximately 10mm in sidewall height. If you are only replacing two, get the same size/brand/tread/model that matches the other two. Volvo designed the car to ride and handle correctly with four identical tires mounted on four identical wheels. If you are replacing all four, go back to the 'standard' size, at least for speedo calibration. If you live in the snow belt, run four identical winter tires (studs/no studs/studless) and your car will drive and handle much more predictably. Mismatched tires compromise the suspension, steering, and braking balances designed into your car. I drive a LOT of cars and have seen some terrible combinations of tires on cars, some with hazardous effects to safety.