I think it's all good, haha. What people mean when they say a graphics card takes two slots is that the card's heatsink assembly is so large that you can't use the slot right next to the PCIe slot. It doesn't mean the card needs two PCIe slots to be installed on the motherboard
About compatibility, you say you have a PCIe slot, so that's the most important thing covered. Another thing would be power; you'll need a PCIe 6-pin power connector for this video card. If your power supply doesn't have one, you can buy a small converter (two "molex" connectors into one PCIe connector). Also, be sure your power supply is strong enough to power the 8800.
Edit: b1gapl, you beat me to it