Same here. VRAM is detected at 494MB in DXDIAG, and 250-something MB in ARMA. Limiting RAM in MSCONFIG fixed this.
This seems to be a general NVIDIA Driver problem as memory detection is borked in DXDIAG already.
i920@3.8GHZ
EVGA Classified x58
12GB PC3-1600
EVGA GTX280 HydroCopper, driver 186.18