What graphics card can I upgrade to?

I would suggest to list some games that you want to play - since your question is quite general. Just having a quick peak at your system, I'd say your cpu will probably be the one to hold you back, not so much the GPU. Depending on your upgrade path, you may also need a power supply (usually prebuilt systems come with a low quality/low wattage psu, you would need to check which exact model you have - as precaution rather)