Hey! Hmmm, well, my game runs pretty smoothly regardless of the amount of CC (I just like to keep it around or under 10 GB for my own sanity/catalogue browsing/etc.). I start noticing a bit of a difference in load time when I get to the 12 GB or higher mark (which is where I am currently… in the middle of a cc cleanout, heh). Honestly, though, it usually always loads up between 20-25 minutes. It may be different for other people, even with similar specs, because load time is affected more by the amount of files (not so much their file size), how deeply you have everything subfoldered, if there are any special characters in the file path, etc. @dramallamadingdang has a post about it HERE, if you’re interested!
As far as keeping the game running smoothly, this was the singular best thing I ever did. My game almost never crashes (even on large lots with a lot of Sims present… maybe a bit laggy, but no crashing!). So as long as you have more than 2 GB of RAM, give it a shot! 🙂