The latest Fiserv Case-Shiller Home Price Index found that homes (on average, nationwide) are the most affordable they’ve been in 40 years.

Fiserv bases this on the fact that the median home mortgage payment is only 12 percent of the median home income. (This is based on data from the Federal Housing Finance Agency.)

Granted, this is about as broad a brush as you can paint with, so it’s only good for giving an overall picture of the US housing market. But it’s a good picture, and it reinforces something that most of us already know: If you can, it’s a very good time to buy.

But wait, there’s more. Fiserv also found that “After years of large declines, the housing market is showing signs of stabilization.” Specifically, of the 384 metro areas it tracks…

18 percent showed home prices unchanged or increasing since Q4 2010

32 percent saw prices declining less than two percent

Based on this, Fiserv predicts “a further modest decline of 0.8 percent by the end of 2012,” but that prices are “expected to rise at annualized rate of 3.9 percent over the next five years.”

Click here to read it straight from Fiserv.