A back up is essential before you do anything, particularly a OS upgrade. You do have one, right? Yes, officially this is a vintage computer - that's Apple description. If your Macbook is 5,1 or newer, and has enough RAM, it will take Yosemite.
But bear in mind that some of your Apps will not work with any OS higher than 10.6.8. That is potentially a more expensive burden than any other you will face in upgrading. A list of > 600 Apps and their compatibility:
http://roaringapps.com/apps?index=a
I have more than one MBP that are older than yours. Here's the essence of one:
I have had OS 10.10 on this computer since the day it was released. But I have also maxed out the RAM, and installed a solid state drive. This computer is fine, actually better than fine. The only drawback I have run into - I imagine there are others I have not encountered - is that I cannot install Windows 8 - Boot Camp Assistant does not want to do it.