It is critical to understand that you are talking to a 'virtual' display.
With 'auto', there will be no RAM physically allocated to the display. If you go to the Windows properties, it'll give the Dedicated Video Memory as 0GB, but then shared Graphics memory as probably 1407MB. If you explicitly allocate 2GB, instead it'll list 2GB as dedicated video memory, and another 1407MB as Shared, and give a Total available as 3455MB. I run like this with the screen at full colour depth, at 5120*2804, without problems. The Auto 1407MB though is fine. Remember that if something like a graphics function is performed, this is done by the Mac display, not the virtual display.