Unless you're a pro-level gamer, you can still play well with 8GB RAM, and 16GB is more than enough. And TS4 isn't as space-hungry as TS4 where saves with World Adventures data would go to 2GB+/each.
The base game takes the most space, packs are much lower - around 2GB for EPs, 1GB for GPs, and 0,5GB for SPs.
TS4 is quite gentle in this department. My TS4 folder (with roughly half the packs) is ~30GB game install and ~5GB of CC+mods. World of Warcraft is somewhere around 100GB at this point. There are games that are getting close to 200GB. If the game seems to be slow, getting an SSD instead of an HDD could be a good idea.
