If you can find a smaller car audio place, buy the deck from them and they can install on the spot, no waiting. Prob take you 30 minutes at most. It's really easy to install stereos, if you know the car and where the screws are to remove the dash area.
If you go with Best Buy or Circuit City, you may have to wait later that day at best or some time in the coming week. I've found the best customer care and treatment with the smaller guys, who have the time to show you everything and can have time to chit chat with you, instead of just trying to close a sale.