And that's why I said ALL options should be mentioned in his FAQ. A lot of them are, but some of them aren't even mentioned.
I'm no whiz, but I'll try to explain this. I could be totally wrong here.
SinglePassDetail means your videocard, while rendering, only does one pass over the detail textures. Earlier cards had fewer pipelines, thus they would have to make multiple passes over a certain texture to get x amount of quality, and this is called, naturally, multipassing.
SinglePassDetail, therefore, would in theory speed up performance on older cards without lots of pipelines (By only doing one pass per render) but the texture wouldn't look as nice, and that's why you cannot use a DetailMax of higher then 1 with this on. (It would only render the first texture, since the second texture layer is done with a second pass.) On a faster, more modern card (If you can run UT2004 with any sort of detail, this means you) it should make no difference, since the pipes on modern cards should be more then big enough to eat the thing in one pass. (Plus there's more pipelines, as well.)