What's Better: VRAM Or GPU Speed

I'm looking at getting a new graphics card. I have set a budget of about $250 US. And I like nVidia.So that puts me in the GTX660 class of cards, pretty much.Now for that budget I see there are cards that have more VRAM - 3GB vs 2GB,or they have a slightly faster GPU speed and a faster shader clock. All the other specs remain the same for all cards in this class. But to get both it looks like I need to bump up to the Ti version and a $280+ budget.What do you think is the best bang for the buck? How important is any of this - or is it just splitting hairs?My primary application is CAD work using AutoCAD.