I agree that you can easily spend the whole week in Paris and still not see everything. That said, if you do want to go to the Loire Valley, Blois makes a good central location. Chambord is nearby, and several other chateaux are within easy driving distance. If you would want to go farther west, Tours would also be a good base. Remember though, you may not be satisfied with just a couple of days in the area. Plenty to see and do, and I haven't even mentioned the wineries.