What motherboard are you using? Typically you have two large PCI-e slots on most non-uber-enthusiast mobos, a 16x and either a 4 or 8x slot. They look nearly identical, but (in my experience) most of the time, 4 and 8x slots don't allow for direct usage of the GPU (usually if you have a speaker installed onto your motherboard for warnings/beep codes, it'll scream at you [pretty sure the standard is long-short-short, may or may not repeat] to let you know that you have a video graphics issue of some sort). Take a look at the solder joints on the sides of those slots. If you count four or eight on either side (just count one side) of the slot, that's not the right slot (you technically don't have to count it, if the joints stop partway down the strip, that's not a 16x slot). Look for the one with 16 (or joints that go the whole way down the slot).