ATI worked with major memory manufacturers to bring the GDDR3 spec to fruition, and according to ATI, the spec takes the existing DDR-400 and DDR2 specs and makes enhancements and improvements to make GDDR3 memory more suitable for present and future graphics processing. Current DDR memories top out at 400MHz, allowing for a theoretical peak bandwidth of 23.8GB/sec on a 256-bit memory interface, which ATI's Radeon 9700, Matrox's Parhelia and 3DLabs' P10 GPU all possess. GDDR3 will start with a clock rate of 500MHz, and according to ATI, the spec allows headroom to take memory clocks to 800MHz, or 1.6GHz effective clock, which would yield a whopping 47.7GB/sec of peak memory bandwidth.
Source: Tom's Hardware