Smart Money has a good calculator for figuring out how much money the bank will lend you based on some basic income information and factor in the down payment you can shell out to figure out how much house you can buy.
I liked the calculator, because it used a fairly conservative ratio for figuring out how much you can afford. The ratio was about 28% of your gross household income. When people ask for my opinion about this subject, i always use the 25% rule. If your gross household income is $4,000, then a conservative house payment is about $1,000 a month. Going a little over this isn’t going to kill you, but when you get in the 35 to 40% range, you’ll start falling into that “house poor” category.