I just bought an Optiplex 760 mini-tower. I was planning to purchase a ATI 5770 1Gb before which I borrowed an NVidia 8600 to try it and I realized that the Graphic card wont fit as it 5 inches wide and the slot width is only 3 Inches (the Steel plate at the VGA output). and the next problem is the Dell power supply is only of 255W.
More aboutoptiplex graphic card power supply
Well, you're not going to find a very powerful low-profile GPU. Here's an option: