Update I had a friend tell me I could get a new case and transfer all of my hardware.. Would this work?
This might be your best option, but I just don't know for sure. If possible, I'd have your friend take a look inside your computer to make sure the components and the rear I/O shield can be removed.
I'd also have your friend bring over a spare standard ATX power supply to see if the system can be powered up using it.
If the answer to both questions is yes, then just moving the components to a new case with a new power supply and graphics card should work for you.
As for just adding a graphics card to your current configuration, you'd probably see a noticeable difference, but I'm just not sure if it's worth the cost. How much money do you really want to drop into this upgrade?