If all you want to do is play Sims games, 8 GB of RAM is massive overkill.
A good place to play around with building a computer is
eCollegePC . Although they will build one for you, it's also a good place to experiment with different components and combinations. I've had them build 3 computers for friends and the difference in the cost of them building one and me building one with the same components was negligible. eCollegePC provides a 3 year warranty. I'm not sure about CyberPower. Or maybe you could check locally about having one built for you.
One of the main advantages to building your own or have someplace build it for you is the LACK of all the vendor specific software, or as it is more commonly known - bloatware.
However, if you want to learn a lot about how a computer is put together, then building one yourself is a great experience. Just be sure if you have a friend help, at least one of you is familiar with the hardware and inner workings of a computer.