Best PC for Sims 3 and 4

@SimsinettySim  I looked up your motherboard, and it should support any graphics card you'd want to install.  A high-end card would be a waste of money because your processor wouldn't keep up, but a 1650 Super or maybe 1660 Super would do fine.  I will double-check with someone else though if you're interested, because there's no sense in spending that kind of money without being absolutely sure.

Even if you're set on getting a new high-end system, your current one plus a new GPU (and maybe a good cleaning on the inside) would be great for someone else who just wants to play Sims games.  Maybe you have a cousin or a friend's kid or something who could afford a new $160 GPU but not a $700 computer.  Just something to keep in mind.

As for the new computer, I totally understand what you mean about not knowing where to start when you've never done this before.  When I first started thinking about custom builds, rather than pre-built systems, I was kind of disoriented and had no idea how to process the information.  I was lucky to have someone who was willing to answer questions and give me links to read, and I figured it out in the end, but that's a difficult thing to accomplish on your own.

So I guess the first question should be what resolution do you want to use, and how high do you want your framerates?  A graphics card that runs Sims 4 in QHD (2560x1440) at 160 fps is a waste if you're planning on using a 60 Hz FHD (1920x1080) monitor.  Many people can tell the difference between 60 Hz and higher, but many can't.  This will determine the graphics card, or at least the range of cards, and then we can go from there.

If you really do want a system you can upgrade over a long period of time, a custom build is your best bet.  Among other details, the better motherboards made for AMD processors now support PCIe 4.0, which is so fast that there's no current hardware that can take advantage of it.  That will change soon enough.  But a pre-built system would likely have a budget board without any extras.

Let me know about the monitor, and I'll have a look at a couple of custom build sites later today.  Then I'll link you the best options I find, and you can ask questions from there.