It's not quite that simple. Memory monitor always stayed green on our Mac, but the difference between 8GB and 16GB was unbelivable, even with only Lightroom and Photoshop open. With only 8GB, having mail and Safari open at the same time led to Photoshop basically hanging while importing from Lightroom, and Memory monitor still didn't say that there was a problem. I've seen older Mac Pros happily chewing through more than 48GB of RAM using Adobe CS.
Not having upgradable RAM is a bit of a show stopper. When our Mac was bought, 16GB wasn't an option from Apple, and was barely available elsewhere. A year later and suddenly the upgrade option was available from many places. Buying a whole new machine just to change the RAM is plain wrong.