just about to buy a gtx 580
but not sure whether to go for 3gb of video ram or 1.5
I will mainly be using this for a 3d setup using 1 23 inch screen
If the extra vram will really make a difference then I will
definitely go for the one with more mem
More VRAM only means more performance in scenarios that are memory limits, generally that means high resolution with AA on. 1920x1080 will be fine on 1.5GB of VRAM even with AA, if you wanted to do 2560x1600 with 16x AA with an SLI setup to give you enough GPU power, then you might run into issues with 1.5GB not being enough, but for most cases 1.5GB will be plenty.
How much is the price difference between them? If its fairly small it might be good to have for the future, but if its like an extra $50-100 its not really worth it.
I want to buy now, I also solely want to upgrade for 3d (more like sidegrade as I have a 6970.)
So it has to be nvidia, I have a 3d monitor thats very expensive and has no hdmi 1.4a connection, meaning that I have no way of playing 3d using my 6970