Are you sure you didn't accidently bump anything when you switched cards? The first thing that I can think of from the top of my head, is that you might have bumped the CPU fan/cooler, and now it no longer cools the CPU as it should. A computer will still work to a certain extent with a non properly cooled CPU, but you won't get too far, as explained by your symptoms.
First thing to check, if that is indeed the problem, take the side of your case off, turn on the computer *with* the side off, and check the CPU fan is spinning. It should be on top of an alumminium block of some sort. If it isn't spinning, find the power lead for the fan. Follow it all the way onto the motherboard (the thing that everything connects into), and make sure it's plugged in. If it is, unplug it, and plug it back in. If it isn't, look for 4 protruding small pins coming out from the motherboard near the CPU area. It should have in writing near it: CPU_FAN, or something to that effect. Plug it in if it isn't plugged in.
I hope this helps

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