I have a Dell Optiplex 760 slim case and I am running an XFX R7-250 2gb (with a low profile bracket), and 8gb Corsair DDR2 ram; kept the stock 255w psu, and running flawlessly for gaming.
You can install any brand of Graphics hard (ATI, or NVDIA) i personally believe that the ATI has better performance over NVDIA, also please check for the motherboards expansion slot, buy a card that matches your expansion slot
I have a Dell Optiplex 760 slim case and I am running an XFX R7-250 2gb (with a low profile bracket), and 8gb Corsair DDR2 ram; kept the stock 255w psu, and running flawlessly for gaming.