Your motherboard can't slowly die. Either it works or it doesn't. Your processor (CPU) slows down as the heat goes up, eventually shutting down completely.
You need to learn how to do those things. If you buy a new computer it will eventually get crapped up too.
Your current computer needs some tender loving care. All computers get dirty inside, that will cause overheating and CPU throttling. It will happen with a new computer too.
If you can't or won't perform basic maintenance then find a shop or a friend to do it for you.
Do you understand the Dell link you provided is for a base computer. You would need to upgrade the graphics and maybe the power supply and other items to play any games.
The case is not standard and not easily upgraded.
If you own a car you need to perform basic maintenance. Same with ANY computer.
Depending on the environment (how clean) you need to clean the insides out at least once or twice a year. Use a compressor or canned air from a office supply to BLOW out the dust. DO NOT use a vacuum cleaner. It will damage the hardware from static electricity.
No one is telling you to build a computer. You need to know what you want inside though. That requires research.
Changing the power supply and adding a new graphics card can be done a ANY reputable computer shop. They can also clean up inside and clean out your hard drive.