I own a Radeon HD 6970 and I use the free drivers (Mesa, Gallium3D, all that good stuff). I really dislike Catalyst, as it's glitchy and causes random kernel panics. But I digress.
The main point is, I fired up Big Picture Mode on Steam the other day and in the video settings it says that my card only has 256MB of VRAM. My card very obviously has 2GB GDDR5 memory plastered all over it. What's the deal here?
I don't know how steam does vram detection, but OpenGL does not provide a mechanism for providing the amount of vram on a GPU. I suspect steam just defaults to saying 256 if there is no mechanism to find out. Rest assured the open source drivers can use the full amount of vram on your card.