Windows XP - Media Center Edition
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.
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