That is a verrrry old computer that was never designed for more than playing videos, it has a cruddy power supply, and it's in a slim case so you would have to find a low-profile graphics card. You could stick a 7750 low profile in there, but it'll be a pain, and useless later.
I highly suggest you save up your money for a new computer - you obviously are looking for more performance, and that's the best way of getting it.