Nvidia knows that ATI can no ship many R520 XT's running at 600/1400MHz and therefore won't get into relaunching a new faster clocked G70 chip. The G70 Ultra was supposed to be a faster version of G70 card based on existing 110 chip, just clocked faster.
Nvidia G70 ultra would likely be very competitive to R520 XT but it would hurt G70, Geforce 7800 GTX sales and therefore Nvidia won't release it. So all of you who have been waiting for 7800 Ultra you can just forget about it.
Business wise it's better to ship many GTX and GTs and sell them in quantities than to release a limited but faster Ultra. We learned that Nvidia shipped a few tens of thousands of GTX and GTs already and it has been shipping in numbers ever since. Last thing you want is to upset people with releasing Ultra, a card that would be marginally faster than G70 and likely to create more problems than solve them.
In that light a very limited but available R520XT might be a Yule winner and best performer and Geforce 7800 GTX is here to stay. At some point Nvidia has to update it to a 512MB memory version but that's about it. The key problem for ATI is that it cannot ship a few tens of thousands of those units until December, no chance at all, and Nvidia can reach and beat those volumes with its highly available GTX part.
Nvidia will have some product refresh of its existing architecture but it's just not the right time for it I guess, it will take longer. µ
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