A/c To my searchig.... ur Pc support graphic card which have maximu width is 7 inch and height is 3 inch...
following are the choices u have
1) EVGA 9500gt
2) Radeon HD 5550
3) Raedon HD 5570
Buy minimum 1 gb memory graphic card
and many more.... which I missed... U can go to any Gud PC shop or Galaxy and asked them to give u graphic card that can be fitted in ur pc.. tell them the height of graphic card that can be supported... that is 7 inch by 3 inch..
and u also need to upgrade ur PSU to minimum 450 watts... ur Pc must have 160 watts psu according to hp website but gpu need minimum 400 watt psu.
All this mean that minimum 20k kharcha.......
If u only want to play with mods than this also possible with XBOX 360 or PS3.. Mod can be downloaded from internet...