2048x1280 for a 1024x640 (HiDPI)
Well, all I know is, they(nvidia and nvidia team in apple) added new mechanism in nvidia drivers onwards the 10.8.4 and later (including 10.9), to check the added or overridden resolutions. I had to revert to the older nvidia drivers from 10.8.3 while I'm being on 10.8.4 and 10.8.5 and it did work. But for 10.9 I'm out of luck. The old drivers aren't even linking on the 10.9… I already tried the nvidia web drivers too.