Hello,
One of the best reasons to build a custom-made computer (may it be yourself or through a customiser like CyberpowerPC) is that you have quite a number of options in the future if you would like to upgrade. If you choose the computer in the link, you can pretty much upgrade or change whichever parts you like in the future.
I would not suggest buying the Intel Core i7 4790K, as this processor is more designed for users who are doing video editing or handling other media, and will not make The Sims 3/4 any faster really. The Core i5 4590 that I have chosen would be perfectly fine for the highest settings in both games. But you are free to upgrade the memory to 32GB or upgrade the graphics card, however you won't notice much performance difference if you are upgrading from 8GB of RAM, and I've already chosen a graphics card that is very powerful. But as I said, you're free to upgrade whatever you like in the future - I would perhaps definitely consider choosing an SSD as well as a hard drive when you buy it, for quicker speeds.
I think for you, Windows 8 will be the simplest option, as the support for Windows 7 will be ending soon. The main difference is that Windows 8 has both desktop and what's known as "metro" modes.
I hope this helps and keep simming,
Cody