When ATI delivered the 4890 video card they increased the clock speed to a blistering 850MHz. With that increase in clock speed came more heat but little was done to improve the stock reference cooler they used. The cooler was really close to the cooler used on the 4870 which had a much lower clock speed, and even at that clock speed still ran too hot for a lot of enthusiasts.
Aftermarket coolers exist for GPU's, but that adds extra expense and it adds risk. Having been around computers since they set them on desktops we've seen a lot of aftermarket cooling ventures go badly, so badly that vendor estimates range from 5 - 10% of aftermarket cooling attempts end in catastrophic failure of the GPU. Improper mounting and lack of good instructions result in GPU death more often than the aftermarket cooler manufacturers would like to tell you.
If your looking for the insane clock speed of the 4890 but don't want to see the 90° C load temperatures common to the 4890 series some form of improved cooling is needed. A much safer option than upgrading the cooler yourself is to buy a 4890 with a factory installed aftermarket cooler. A pre-installed aftermarket cooler means that you don't chance killing your GPU, and the factory warranty won't be voided by installing an aftermarket cooling solution.
Enter the Gigabyte 4890 OC with it's pre-installed aftermarket cooler and blazing core speed of 900MHz. You get the best of both worlds, an aftermarket GPU cooler, and factory overclocked while keeping your warranty intact.