Since 2.73 version Blender 3D/CUDA/Cycles is using about 800MB of extra vRAM in any scene, even with the default cube. Supposedly this is related to new versions of CUDA that are used to compile the kernel in Blender/Cycles:
Blender 3D developers have commented that perhaps the use of vRAM will be reduced when the AMD/ATI Kernel Split arrives to CUDA:
But that may take a long time until it happens. Currently on cards with less than 2GB of vRAM, Cycles/CUDA can be practically unusable.
So from nVidia side: Can you do something to try to solve this problem?