run CPUz as noted above, then lookup motherboard type. Google motherboard info. The three common types of GFX interfaces are PCI-e, AGP, and PCI. Dell Fvcks most of it's customers over by leaving them with PCI only.
well one thing to find some computer specs is right click on my computer icon go to properties and it will have it listen near the middle-bottom right hand corner, otherwise there should be a label on your computer if it was bought already assembled.