Looking at eBay, there are 3 motherboard configurations. You said that that you have a PCIE slot so that narrow's the options down. I can't find a picture of the inside of your (mini ?) mid-tower case. As Gareeee said this card will use the PCIE slot and cover your remaining PCI slot. With that in mind you will have to add extra cooling or both the CPU and video cards are going to over heat, giving you worse performance that you are getting now.

Just from a personal point of view, your Dell Optiplex GX745 does not suck. It was built for a specfic purpose. Your knowledge and abilities have out grown that purpose. Yes it is agravating, but is really not Dells fault. The type of Dell you sould be looking at now would be the XPS series, which is a higher end desktop. Don't put much more money into your GX745, as I am willing to bet you will find the processor and or RAM will be your next bottle neck.