OP, you have a prebuilt. You could upgrade the vid card and might need to upgrade the power supply.
It would probably be a better move to just build a new computer. Mainly because all the newer builds, require a new motherboard, RAM and CPU.
If you decide to do this... please provide the following information:
Budget:
More specific use (what games, what programs):
Planned time until purchase:
Do you need an Operating System?:
Do you have any specific preferred components?:
Do you need a sound or wireless card?:
Do you want stock cooling or do you plan on overclocking?:
Do you need a monitor? mouse? keyboard?:
Zip code/location? (Shipping costs):