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zarx said:

So unless you have a source that 4Gbit GDDR memory chips will be available in the next few months.

Are you kidding?

2008 news... "Presently Qimonda has 512Mb (16Mx32) GDDR5 chips at 3.60GHz, 4.0GHz and 4.50GHz clock-speeds in PG-TFBGA-170 packages in production."

http://www.xbitlabs.com/news/graphics/display/20080510113121_GDDR5_in_Production_New_Round_of_Graphics_Cards_War_Imminent.html

The PS3 uses 8x512MB GDDR5 (or 4Gbit like you said) to get ~170GB/s.... GTX 680 uses 16 chips because they need a 512bits bus width for near 380GB/s brandwidth... it's easy to make maths.

8 x 32bits = 256bits bus width = 190GB/s at high speeds
16 x 32bits = 512bits bus width = 380GB/s at high speeds

nVidia choose 16 x 256MB chips because they need to reach 380GB/s bandwidth... not because there is no 512MB chip.