It was a fresh install of 10.7.5 that I downloaded from the App. Store.
What sort of graphics card do you have? Do you have more than one card? Are they matched? Did you try checking your Preferences for Graphic Processor Settings in the Photoshop CC Performance preferences? The reason I ask is that that Photoshop CC has extended the use of GPU in a lot of areas...one are is in Image Size and in Smart Sharpen. As far as I know, the GPU is used to speed the results if the card supports it. In a 2006 MacPro, that card is ancient by comparison with more recent computers...It would be useful to compare the GPU settings between Photoshop CS6 (which is less reliant on GPU) to the settings in Photoshop CC.
I'm running Photoshop CS6 and CC on a 2009 MacPro with a pair of matched NVIDEA GeForce GT 120 cards (not very powerful by today's standards) and I'm running the GPU with set to "Normal" in both CC and CS6, I'm getting the same sort of speed between CC & CS6.
in fact, it would be useful to make sure ALL the prefs between CS6 and CC were set the same. 8 gigs of ram isn't a lot and you should think about increasing the ram. Also, where is the scratch disk set to in CS6 and CC? By default CC would be using your boot drive unless you changed it after the install.
But none of this would explain why your system was reporting less installed ram after you installed Photoshop CC–which should have no impact on the system's installed ram.