"It's a brave, new world with respect to credit requirements for mortgages," says John Ulzheimer, president of consumer education at smartcredit.com and formerly of FICO, which pioneered credit scoring.

One old rule still applies: The higher your credit score, the lower your down payment and monthly payments.

"Below 660 or 680, you're either going to have to pay sizable fees or a higher down payment," Zigas says. And that's pretty much the cutoff score for getting a mortgage, he says.

Vicki Bott, deputy assistant secretary for single-family housing at the Department of Housing and Urban Development, says that her office has noticed much the same thing. "While there are many qualified borrowers in the 580 range, the market today is probably (looking for) 640 to 660, at a minimum," Bott says.

On the other end, a score of 700 to 720 will get you a good deal and 750 and above will garner the best rates on the market, Ulzheimer says.

Improve your chances by: pulling your credit reports and ensuring you're not being unfairly penalized for old, paid or settled debts, Zigas says.

Stop applying for new credit a year before you apply for financing. And keep the moratorium in place until after you close on your home, Ulzheimer says.