If your machine does not support Mountain Lion and you want to go to Lion, you will need to call Apple at 1800MYAPPLE and for $19.99 they will sell you a code that can be entered in the App Store to DL Lion.
My late 2008 Aluminum 2GHz MB runs admirably even with 10.8. You just have to ensure plenty of RAM and HDD space are available so virtual memory isn't throwing a wrench into the speed. I don't see why you would want 10.7 over 10.6 though. I think I upgraded to 10.7 in the past but nothing big. I think I'm stuck in a 10.6 mindset.