The official GT300/Fermi Thread

[largon]

Maybe AMD should start using GDDR3 or GDDR5 for desktop. With a couple PC3-1600 DIMM, you only get 25.6GB/s. 5870's GDDR5 would give you 80GB/s - 3x more.

Answer this:
Would you choose 1GB of unnecessarily fast (as system RAM) GDDR5 memory, OR 4-8GB of plain-jane DDR3?
No need to answer, you would go for capacity.

And why should the HW makers even bother considering?
System memory bandwidth has basically no effect on system performance. Capacity, or rather the lack of it, has HUGE negative effect. Then there's the huge bunch of issues starting with cost difference between DDR & GDDR, multiple times more complex buswidth, capacity as mentioned, power-thermals and motherboard trace lengths.

GDDR4, GDDR5 (...) are quad data rate hence why 5GHz is so easy to reach.

No, they both are dual data rate, just like all DDR before them.

and GDDR6

GDDR6? What's that?