If you're doing just regular 2D desktop work, yeah, the D300 is plenty capable.
However, early tests of any 3D gaming on 4K has shown that 2GB creates issues and 3-4GB of VRAM is much better on 4K. This may be telling about how much VRAM would be needed for professional 3D work on 4K as well. IMHO, It would seem like a no-brainer to go for the D500's added 1GB of VRAM if you plan to use 4K displays... it's a $400 option.
If you wanna do any gaming on 4K with high details you need D700s. With the D500 the extra ram probably isn't gonna help because the chips are not fast enough to render textures that require that memory.
(but keep in mind the d700's are only around 30% faster the d300's)
D300's aren't slow they are roughly equivalent to 7970m crossfire under windows. http://www.notebookcheck.net/AMD-Radeon-HD-7970M-Crossfire.74305.0.html. (around same clock speed, same pitcairn core)
If you do 3d rendering then the extra memory is welcome. But again if you do any kind of serious work you're probably much better off with the D700s (which have a serious amount of VRAM).
I would either go D300 or D700, depending on your needs. The D500 aren't the best value unless you have specific needs and a very specific budget.
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