Unless you purchase a very low end card, you are going to need a new power supply also. HP psu's are very weak, and do not do well with anything over an HD 7750 or GTX 640, both of which are pretty much entry level gaming (the 7750 is the best you can buy in that category).
In order to run a low-mid range card such as the HD 7790 or GTX 660, you are going to need a quality 500w psu from a reputable maker such as Corsair (HX or TX), Antec, Seasonic or XFX.
Mark