Thing is, you answered none of my questions, and without any of that information nobody can tell you what could be safely done without making your motherboard potentially sizzle a firey death. Not to be overly dramatic or anything, but DELL specifically has put out all kinds of non-standard stuff, and there's no way for any of us to just know what it is. And to make it worse, some of their non-standard power supplies are pin/plug compatible with the standard so if you accidentally mix a normal power supply and a proprietary motherboard (or vice versa) you could totally fry both.
If we knew that it was standard at the very least we could then use info about the CPU and whatnot to pick a matching video card, and since it's a slim case just tell you to buy a normal sized case. (if indeed a slim version was just too expensive or whatever)