Note, MacBook Pros newer than October 26, 2007 may need to watch out which 10.5 installer disk they get.
Apple released 10.5.1, 10.5.4, and 10.5.6 as retail releases. Given the fact that Apple has 10.6 available and it is compatible with more applications than 10.5, and has more current security updates available, I would say that for MacBook Pros it is far better to purchase 10.6, and bring it up to 10.6.8, than buy any version of 10.5. Only a few applications can run on 10.5 only. Everything you needed to know about 10.6 is here:
If you don't rely that heavily on old software you may want to jump straight to 10.7 or later after that: