Well, i did a small research on the net. I had 2 GPUs in my mind. First is a AMD 7850. But it requires a minimum of 500 watts PSU. It might work with your system but i really don't know what kind of PSU your system has.
The second card is the Nvidia GTX 650 Ti Boost. According to Nvidia's product page, this card requires a 450 watts PSU. So this may fit better in your system. This is a great card that requires relatively low power and it is decently fast for 1080p gaming.
But the ultimate question is, will a new GPU fit in your case? You should know this before making a purchase
Thanks for your reply. I'll check my system again for the details. And will reply you soon.