Fokissed :
A 256 bit bus doesn't suffer from a portion of slow VRAM. A 256 bit bus needs 1, 2, or 4 GB of VRAM while a 384 bit bus needs .75, 1.5, or 3 GB of VRAM. anything else will have a slow (reduced bandwidth) portion of VRAM.
That's more of a compatibility issue, and I never mentioned the
speed of the video memory, I was talking about the
effectiveness of it and what amount of bandwidth will actually be used.
A larger memory bus
DOES have a profound effect on the memory bandwith of a graphics processing unit, otherwise it wouldn't make sense to have anything higher than 128-bit or 256-bit memory buses, yet you see R9 290's and 290X's, as well as GTX 690s (just to name a few) with 512-bit buses. The simple truth is that
higher amounts of video memory require larger memory buses to be fully utilized. Just take a look at any benchmarks, wether it be the one I linked or another on the world wide web, it is conclusive that at a whole bunch of different high resolutions, the 4GB edition of the 770 outperforms the 2GB very inconsistently, and when it does, it is usually 0.1-2FPS. In my opinion, that is nowhere worth spending an extra $40-$100, which could be used towards a better GPU or processor etc., but if that is what you really really want, I guess go ahead. I would
highly advise against it though.