Mountain Lion actually seems to be a bit less memory intensive than Lion. Whether that's by design, or just a function of the general disarray Lion was in for its launch, you shouldn't worry about RAM issues.
"Apple's OS X Mountain Lion is out, and it's 20 bucks. That's cheap enough that the price alone probably isn't a barrier to upgrade-it's more about your individual needs. Here's a quick guide to see, at a glance, if you should go with Mountain Lion, or maybe sit this one out."