I have not heard a 5800 (ultra or non-ultra) that is not irritatingly loud, especially when idle. The 5900NU 400/700 isn't noticably slower than the 400/850 model, you might as well just stick with it. The 5900NU will toast the 5800 on DX9 games and if you use AA/AF.
We're all in it together.
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