Originally Posted by dragosmp
GDDR3 is basically DDR2 with loose timings and 2V stock voltage.
GDDR4 - not sure, but I think they are still based on DDR2 technology (since the frequency range isn't that much changed and the voltage is pretty much the same).
GDDR5 is sampling from Samsung and it's likely it's based on DDR3, but that's just supposition.
Comprehensive differences of GDDR3 vs GDDR4.
GDDR3 vs GDDR4
1) 7.2 vs 11.2 GBps per chip max data speed
2) 1.8 vs 2.8 GBps per pin max data rate
3) 1.8 vs 1.5 v minimum power consumption which results in a 20-25% energy savings
4) 2.1 vs 1.9 v maximum power for overclocking
5) 4 vs 8 bit burst length which allows more memory bandwidth and lower latency
6) no DBI vs DBI: Data Bus Inversion - it takes more energy to transfer 0s than 1s. When there are a lot of 0s to move, it will send them as 1s instead and then use DBI to signal that all those 1s needed to be flipped back to 0s. This is a power saving feature.
7) Three point voltage control via video card driver for fine tuning and overclocking