The Inquirer claims to have definite word that ATI's upcoming 80nm R580 die-shrink, the R580+, will use GDDR4 memory. Current graphics cards use GDDR3 at effective speeds of up to 1.7GHz, which allows for 6.8GB/s of memory bandwidth per chip, and 51.2GB/s of total memory bandwidth for cards with 256-bit memory interfaces. GDDR4, however, has been pushed to 12.8GB/s per chip, which translates to 102.4GB/s of bandwidth on a 256-bit memory bus. Samsung began sampling GDDR4 RAM back in October last year, and stated that mass production would follow "by the second quarter of 2006." Considering the alleged June-August production date for the R580+, everything seems to match up. If the rumors are true, it'll be interesting to see just how fast ATI will be able to ramp up GDDR4 memory on production hardware.
