My Alienware Aurora keeps slowing down..?

Hi, I'm new to this forum.

I bought an Alienware Aurora about a year ago. When I set it up, there were some issues with the graphics card because of which the icons/start bar won't show up. So I contacted Dell support and they took my computer to check. They changed my graphics card. Everything was alright for a little while but recently I have been encountering some new issues. After booting up, my computer automatically slows down after a while, even when I'm not working on a heavy program. 

I think it may be due to incompatibility issues caused by the new card. I read up on the Internet that the most compatible card for Alienware Aurora is Nvidia GeForce GTX 560 Ti, and that one I have is GTX 460.

Do you think this might be causing problems? Should I get it changed?