If you are going to spend 1000 on that Dell you should have a rig built for you. I went through Costco and bought a Dell Insp 580. I tought it would handle gaming. I was wrong. The main problem you are going to have is Heat. They are cramp inside. And also the PSU are not good they will tell you that it is 460w you will be luckey to get 400w I had a lot of problems with mine. So I went ahead and built one. I put only 1400 into it and I love it. It does everything I need it to do.
Now if you are intent on a card The 560 ti is a good one but you need a 6 pin coming from the power supply to run it. The 560 requires a 450w power supply and up. If you want a cheaper card go with the ATI/AMD a 6770 card. ATI cards are best for lower end cards for the money where Nvidia is more for the upper end cards. I hope this was a little help. Good luck