9800 GT may be simlar, but it is still more advanced in some ways. The geforce 8 series was one of the first DX 10 graphic cards and was miles ahead of ATI at the time. I would not be suprized if the first DX10 hardware was not the most efficent and suffered from major inefficencies performing more complex opperations.
Although the 8800 GT was a remake of the 8800 GTX/GTS with faster interfaces and improved efficency due to smaller die size, it probably still suffered the same early DX10 efficency flaws that the first generation of cards did. However because it was more energy efficent due to smaller die size and was meant to be a cheap card (as it was released around the time as the 9800 cards), they stuck reduced cooling on it compared to its predecessors. Combined with expensive inefficent DX10 opperations, this would explain how it can overheat at 100% fan speed as cooling reduction was based on average power consumption and not peek.
The 9800 GT was a generation more advanced and so probably had additional optimizations on it. Its larger size and power also meant more cooling was needed and its expensive price would allow for that.
My 275 GTX card has no overheating problems in SC2 at all. Ultra all the time to. I am suspecting my brothers brand new 460 will also not suffer overheating at all.
Thus I blame overheating to design faults from the vendors and to initial teething problems with the technology. Like the Xbox 360 with its reduced power and no overheating lite model, time allows for improvements in nearly every way on nearly any hardware system of a large scale (hundreds of millions of transistors).