The specs don't state which PCIe x16 slot you have. There have been several revisions since the original (version 1.0). If you get a version 2.1 PCIe graphics card, it may not work with earlier slot versions. You also need to check your power supply specs (amperages, not just wattage) to make sure it can handle a card, especially if you're looking for a gaming card.
See this article: Best Graphics Cards For The Money: May 2012
From that article: "The Radeon HD 7750 is the fastest graphics card you can get right now that doesn't require a auxiliary power input; it draws all that it needs from a 16-lane PCIe slot.
If you're upgrading an older machine with a power supply limitation in mind, this point warrants an honorable mention."