There is a lot more than just the CPU that needs to be confirmed before you can add a graphics card to your system.
1) Does your system have a PCI-Ex16 expansion slot? If not, then it's unlikely you'll be able to add any card.
2) Is there enough room within the case to physically fit a dual-slot, full height graphics card? If not, then you will be limited in what cards you could install and the cards available may not give you the performance you're looking for.
3) Does your system have a power supply powerful enough to run a discrete graphics card? Most entry level cards can be run on lower-powered power supplies (250-350 watts). Anything lower would likely need to be replaced. The more powerful the card, the more power it requires from your power supply.
3a) If your system currently has an underpowered power supply, is it replaceable? Some small form factor PCs have power supplies that are proprietary and cannot be replaced with something significantly more powerful.
If you could give us the full specifications of your computer or if you purchased it from a store, the manufacturer and model number, we could help you get a clearer idea of what you could do.
-Wolf sends