PC turning off and fan blasting whenever I get in a game

Computer was working fine for a while after I got a new graphics card and a new ram stick. I have a Dell 5g gaming desktop. The new card was nvidia something, I used to have an amd something. If the numbers really matter I can go look at them, but the Nvidia one was better than my old one.

About a week ago, whenever I would play games about 10 minutes in it would shut off the monitor and blast the fan. I updated the drivers, cleaned it out, that didn't work. I did a memory scan, it found nothing. In fact, now it instantly does the thing when I get in a game So I just tried to remove the new ram I got. And that. Didn't. Work. I'm going to try to put the old graphics card back in and see if that works. Please help me, anything will help.