Dells have custom Mainboards specifically for Dell PCs. Basically, you can't use them very well in non-Dell cases, as the Graphics Cards never line up with the holes in the back of the case, and the screw holes are in a different place to standard ATX. I have no idea why they do this. To conclude, you should probably get a new mainboard. Dell use an unbranded Korean mainboard of some sort (I'm too lazy to look for it in my pile of computer spares), which is alright, but not the best quality in the world. Go for an MSI or something similar.