maybe you better wait for this http://www.guru3d.com/news_story/6gb_ve ... anned.html
for me, i almost always use more than 3 gb (and even not all 3 gb are available on a 3gb card, a few hundred megs always gets lost or by the display, or any other reason)
i do complex exterior architectural scenes with many objects and textures.
If your work doesn't need complex scenes (studio shots of objects like jewerly, or any other single objects scene) than 3gb will get you there, but with more complex scenes (an also if you need big resolution renders, the bigger resolution, the more vram consumtion) you need more ram.
hope that helps a little.