I have the Asus 5800 FX and the fan does indeed speed up. However, it is not that noticable unlike the flow fx. My temp seems to stay around 50C when playing games. The fan whirrs real loud then immediately spins down when windows starts to boot. The 5800 fx is a nice card. I'm willing to bet there won't be that much difference between the 5800 fx and the 5900 non-ultra.