Found this and if its right click here you have a PCI Express slot and that gives you a lot more choice as to what to buy.
Changing cards is not that differcult but you do have to bear in mind that more powerful cards require more power and therefore you have to bear in mind whatever power supply is fitted.
As to what you should buy it depends on three things.
One the rating of your power supply although Dell have some strange way of working out their wattage and it might power the card you pick perfectly despite it perhaps being rated as too low.
Two the games you wish to play and
Three how much you are willing to spend.