What genre of games are you interested in? The graphics card you'll need will be very dependent on that.
The machine you have supports PCIe upgrade, which as far as I know is still the standard, however you won't be able to use SLI or Crossfire technology, as from what I can gather, it's only got one of such slots for upgrade.
So, to get top-notch performance, you're going to have to pay for a single powerful card. As opposed to combining mid-range cards a la Voltron.