There have been cards in the past that used RDRAM, so I've little doubt it's possible. I can't see WHY you would possibly want it though.
Many, if not all of RDRAM's benefits are wasted on a video card, and when utilized in the manner necessary for graphics DRAM it would provide bandwidth significantly lower then modern DDR SDRAM on a 128bit data path can provide.
Remember, system DRAM interfaces and the memory controller on a video card are vastly different things. RDRAM would be a poor solution at best.