If the rumors are true, this will be more like a L2 cache.
The cache is supposed to work as a general purpose buffer between the chip or texture cache and the "slow" on board RAM of the gfx card. This is based only on rumors that have been floating around. Amount and speed are all over the place along with type, some are saying eDRAM and others are saying SRAM, some are saying full clock speed, others are saying full clock DDR(double the clock) and some are saying full clock QDR(quadruple the clock). So much speculation out there and nothing solid from nVidia to indicate that any of it is right.