You are crippled by two factors. The first is that any "good" graphics card will almost certainly require a new power supply. The second is that you have a slimline model, meaning you'll need a video card that is able to accommodate that particular form factor. Even if you moved everything to a new case, you'd still need to add to that a new PSU, so it would become a significantly more expensive proposition. My personal opinion would be to not even bother, just save your money and buy/build a new gaming oriented computer when you have more money. If you try moving the contents of that computer to a new case would likely result in you replacing significant portions of the computer anyway.