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I quickly tried it under Vista 32bit and XP 32bit.
Under Vista Nvidia preferences had trouble getting it to change resolutions, but the Windows and Dell control panel had no problem at all and once you lock it in with either of those it's good.
Vista seemed to have a few more occurrences where things are not aligned up quite as well once 200% scaling is on. The Vista desktop with 200% scaling also spaced the icons on it ridiculously far apart, it seems to be a bug where they accidentally scale the icons to be 2x farther apart in each direction and then apply a 2x interpolation in distance between on top of that . So it kinda spreads icons over the desktop in a bit of a wasteful, messy too far apart looking way. OTOH the log-on screen on Vista runs at UHD (on W7 the log on screen, for some odd reason seems to load as a small centered 1600x1200 no matter what, perhaps it's a sanity check feature on W7 to make sure you can access log-on buttons no matter if something weird got set to display settings?).
I didn't try too hard yet, but with Vista and W7 if you use res smaller than UHD but as large or larger than about 1600x1200 it seems to just center a tiny window on the center of the monitor. I'm not sure why no matter what I try to set in Nvidia scaling preferences it doesn't seem to scale it to fit the screen. Maybe Nvidia has the scaling broken once again in their drivers or maybe I need to do something else. I haven't looked into it too much yet.
Interesting, on XP, whatever mode you run scales to fill the display, so if you go to 1920x1080 the UHD monitor simply acts like a 1920x1080, each pixels turns to 4 with no nasty interpolation. So with XP you easily run the display as either full UHD or regular HD, no problem. I have no clue why Vista or W7 don't seem to be like, at least not with Nvidia graphics. I didn't look into how XP handles 200% scaling in UHD mode yet. At first HD was looking so not crisp after having gotten used to UHD that for a second I thought it was interpolating 1920x1080 to UHD instead of 1->4 box scaling but it wasn't it was just doing a straight 1 pixel to 4 box scale, perfect.