5900s = nv35
5800s = nv30
A 5800 Ultra is kinda different from a 5900. Basically, it uses 128bitDDR2, whereas a 5900 uses 256bitDDR1, it runs at higher clock-speeds and also shows some differences regarding PS precision (to my understanding, fx and fp modes used are not exactly the same on both cards...).
A 5800U is a rather rare card, I'm surprized to see BFG (or any other brand) is still selling it.
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