Well, part of me thinks it's just about economics, yeah sure, they may release a massive video card, that will cost $800 will people still buy it? When there are also video cards which may not be as fast (Say Radeon 3870x2), only about 6/10 as fast but is 1/4 the price of that "fastest" video card? Making it better "value" wise?
My mom and me always agree that a good performer with a cheap price will net you more profits than a product is that is the best performance but has a huge price as well. People just buy the cheaper ones nowadays.
Besides, they still haven't broken their product cycle, the 9800GTX is still about 10% faster than the 8800GTX. Usually......
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Current PC specs :
AsRock 945GCM-S
Pentium Dual Core E5200 @ 2912Mhz (233x12.5)
Palit Radeon 4850 Sonic 512MB gDDR3
2x2048MB Kingston DDR2 800Mhz RAM