Which version of photoshop is compatible with my Mac OS X v.10.6.8? Should I use Photoshop CS4 Extended or CS5 Extended?

I have never upgrade my Adobe applications beyond CS3 and they have worked on both Snow Leopard and Lion.

Allan

CS4 Extended is fully compatible with SL. I don't know about CS5.

I've got Photoshop CS5 extended installed and working perfectly on Mac OS X 10.6.8.
and by the way my MBP is a 13'' midd 2010.

CS5 works fully on my Macbook Pro, but it all depends on your specs. If you have a newer MBP on SL, you should be fine. Photoshop isn't terribly demanding on the computer so I wouldn't worry too much about it. Whichever one you can get is fine.