What would be the best graphics card for me to get for my systemax computer model snc302eeh?

I would like to buy a new graphics card for games like Fable: The Lost Chapters. I've looked into NVIDIA and ATI, but I'm pretty sure my computer doesn't have a PCI-Express port. I looked and it just said "PCI Bus". Is that also compatible with AGP graphics cards? Anyway, I just need to know the best possible graphics card for my computer. Thank you for your help!