You might also note that, unlike the more expensive iMacs, this one does not use dedicated RAM for graphics memory. This, lifted
from Mac Tracker:Graphics Memory Up to 1.5 GB shared from main memoryNot to worry, though. I was getting by on 4 GB RAM until just a few months ago with no difficulty at all. Mac OS X, with its Unix underpinnings, can get along swimmingly with limited available memory. It will grab memory space from the hard drive and use it as virtual RAM. The downside is a lower response time. However, 8 GB will do quite nicely. Just remember to use "Quit" from the File menu, instead of just clicking the window closed with the red chiclet. That will clear programs from memory when you're done. (Note that some programs clear when you click the red chiclet, but not all do.)
When I noticed a slowdown on upgrading to Mountain Lion, I upgraded my 2008 iMac's RAM from 4 GB to 6GB, which maxed it out. It quit spooling to HD and performs much better. It is running my WeatherCat, 24/7.