GDDR3 is the most common RAM in video cards currently with potentials of speeds above 1GHz, but newer games and intensive applications have proven that going far beyond that is beneficial. Samsung is right on top of that with samples of GDDR4
that operate at an absolute blazing 3.2GHz (DDR speed). Based on an 80nm process with a 512Mb per chip capacity, they aren't quite ready for the market, but Samsung is preparing. GDDR4 hasn't been officially standardized by JEDEC or any organization, nor are proper platforms in place for utilizing it, so it may yet be quite some time before we see this in cards.