As Linc Davis points out, it could be coated with dust. I'm sure not all video card construction is the same, but since the fan on yours is at the end opposite the back of the Mac, it's likely blowing air through the long portion and out the back to cool it. Inside are heatsinks, sometime tubes, that the air travels through as it heads to the rear, taking the heat with it. It's possible that those tubes are being clogged with dust.
One solution is to use canned air with a flexible nozzle inserted between the fan blades and pointed to the rear to blow the dust out. If the tubes are arrayed like organ pipes, you can move the nozzle along the rear arc of the fan's opening and watch the dust blow out the back. I did this yesterday to an ATI 5870 for the second time in 4+ years while changing a CalDigit PCIe card and a lot of dust was blown out. Since most air can nozzles aren't overly flexible, I attached some lawnmower fuel line with a 3/32" ID (from Home Depot) to reach in without harming anything and with enough of the tubing, the air can can be kept straight up. Just don't shake the can first. 😉
