How much house can you afford?

It's the season of life. Weddings, budgets, babies, and buying houses.
How much mortgage can you afford? I ripped this tip out of Money magazine:

Multiply 25% of your monthly income by 186.28

Then, to find out how much down payment is needed to qualify for that mortgage, multiply the result above by 0.25
Now we're talking.
A recent home-buying friend recommended two sites for house shoppers:
http://www.zillow.com/
http://www.ebby.com/

I'm particularly fond of Ebby's polygon mapping system.