im in the same boat, my mac pro has 2x the standard NVidia 256 card and how sweet would it be to give it at least 128 or a full card's worth and then the other card will take care of OSX.
AVID still thinks i dont have a video card or 64mb just aint enough.
Knowing parallels it'll be another couple updates away before you can feed it more VRAM.
The 3D support in Parallels is not limited by the amount of VRAM assigned to the VM, the memory management for the 3D is done by the OS X OpenGL driver. So the VRAM setting really only affects what resolutions and color settings can be used.