The G71 has no officially specifications announced yet therefore it is wrong to assume it will "double" performance or anything similar. Thusfar, the only conclusive information known about the new core is the fact that it beefs up the already existing G70 core to include 32 pipelines and most likely higher internal clock frequencies
That said, nvidia is taking a rather silly approach to GPU development at this stage, ATI muchlike what AMD has done has redefined GPU architecture and increased pixel operations to three times that of the normal clock cyle. In other words, this means ATI cards won't have as much raw power as nvidia cards, therefore in pure speed situations an nvidia card will start to take the lead. However, once you start throwing HDR and AA/AF and other shaders into the mix, the number of pixel operations start to play a factor as detail come into mind.
Basically, in the case of nvidia, the pixel will have to pass through the pipeline over and over again in order to achieve the desired number of pixel operations, this is normally the bottleneck issue with video cards. ATI has instead decided to keep fewer pipelines, and muchlike original speculation regarding the r520, each pixel pipeline is effective 3 times as powerful as your normal pipeline as it performs roughly 3 times the number of pixel operations.
So, for those of you who supported the "SM3 is everything" ideal of the last year when nvidia were gods because they were the only GPU to support the instruction sets, you should set your sites on the x1900 as it's a much better card for the job than the G70 or G71 for that matter unless the G71 has ridiculous pipeline counts and high internal frequencies
If you don't believe me, look at FEAR, terribly optimized because each pixel contains a huge array of different shaders from shadows to particles. The x1900 cards outperform even 7800 "ultra" cards in SLI with AA enabled
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