First thing is GET A GUIDEBOOK!!!
Prague is the one city I've been to that I neeeded a book. You could spend 2 days just wandering around old town and the jewish quarter. The castle is between a half day and a day depending on how long you stay at everything.
We stayed at the hotel Julian. If you choose to stay there make sure you get a room in the back. The front is pretty loud with trucks and cars. There is a restaurant called the Olympia near the hotel. We stopped there three times for meals and every one was better than the last.
Beer is cheap!!! I mean real CHEAP!! For 10 euros you could drink most of the night and leave with change.
Have a great trip.