Yosemite or Snow Leopard for 2008 MacBook? I have a 2008 MacBook which is getting slow, I need to do an erase and reinstall. 

I have the choice to install Yosemite (online) or Snow Leopard (from a DVD). 
But would Yosemite be too much for my old hardware? In which case could I easily get more RAM put in? 

If go back to Snow Leopard, will that soon be completely outdated?
I am currently using Yosemite successfully with a very similar computer: 2008 iMac 2.66 C2D. I would strongly encourage you to upgrade to 8GB of RAM and swap your drive out for an SSD. After upgrading the RAM to 6GB and making a roll-your-own Fusion drive (I installed an SSD in the Optical drive bay), Yosemite runs very well. Unless you need to use Snow Leopard for Rosetta (i.e. PowerPC compatibility), I'd go with Yosemite.