Cycles GPU problems on GTX 780 6GB edition

Recently i have come up against a problem where Cycles reports the out of CUDA memory error. And yes, i know what that means. The problem is that the scene takes about 2 gigs of memory on CPU and refuses to work on GPU, despite it having memory to spare (4 gigs). To sum up: Cycles, it seems, thinks that my GPU only has about 2 gigs of ram. :no:

What Bleder version? Is it custom build from latest trunk, downloaded from other site, or official 2.71, etc? When you first time see problem, trying new scene with unchanged Blender or get new Blender with old working scene ? Did you updated CUDA related system library (NVidia driver) ?

Sorry. Completely forgot to include this sort of info. Blender version is official 2.71. My problems started today actually. I’m working on a really big scene at the moment, has a lot of foliage and such. I did try to completely reinstall my GPU drivers but it didn’t work. I used a special uninstaller to completely remove any trace of the drivers. I also tried to use an old version of Blender (2.68) and the latest build from graphicall (2.71.6). The result was the same. After some testing I may have found what is happening here. Since the scene file is so big, it takes most of my entire ram to run it (i have 8 gigs BTW), and when it comes to building the BVH there simply isn’t enough space to acomodate both the scene in the viewport and the BVH in the memory, therefore it can not dump it in the vram of the GPU. At least that’s how I understand what is going on.

Just got a total system crash. Screen went totally black and unresponsive. Sigh. It seems that I need more RAM.