If you look on the MacBook section of Everymac you can not only see the specs but see the maximum version of the OS each one can run...
http://www.everymac.com/systems/apple/macbook/index-macbook.html
So if you want something that you can keep for a few years, assuming you would like (or need) to stay on the current OS (10.8) or close to it (as Apple drops support for these older MacBook's in future OS releases), the late 2008 Unibody MacBook's can still run 10.8, and everything newer than that. The Unibody MacBook's look like MacBook Pro's and were only made one year, then Apple switched back to the white MacBook case. You do need to look closely at the various models to see what can run the most current OS, because there were previous MacBook's with the same clock speed that cannot run the current OS. Example, the late 2008 Unibody MacBook's had 2.0GHz and 2.4GHz versions, they can run Mountain Lion. But there were 2.0GHz and 2.4GHz versions in early 2008 and they cannot support Mountain Lion.
If you are looking for a MacBook Pro,
http://www.everymac.com/systems/apple/macbook_pro/index-macbookpro.html, the mid-late 2007's and newer can run 10.8.
But if you want to hopefully be able to upgrade to whatever comes after 10.8 you should probably try and not buy the lowest/oldest model that currently supports 10.8, and instead buy something newer, as Apple will likely move the support requirements up a full model year with each new OS released.