The Aerosoft Airbus X is OK, but it doesn't simulate all systems to a standard which you might want for a 'full' simulation.
Keep an eye on the Flightsimlabs A320 in development which looks like it will be very good. They made a stunning Concorde for FSX and if you have a look at their website and forum, you'll probably be quite impressed at the amount of detail they are packing into the aircraft.