It seems that the G70 / GeForce 7800 GPU  is a very large chip - perhaps 
larger than what would be expected from a 24 pixel pipeline part.

The die size of G70 is  330mm2   - the speculation is, this is consistant 
with a 32 pixel pipeline part.   It seems that the current G70 ~ GeForce 
7800 is not fully utilizing all of this space, in order to keep chip yields 
high.  Nvidia can sell G70 GPUs with only 24 pixel pipelines working, and 8 
deactivated or defective.  for now, it is about having redundancy.  much 
like the Cell microprocessor for PlayStation3 only  making use of 7 out of 
the 8  SPE cores.
chips that do not have all of their pipelines or cores working are therefore 
not garbage but can be used in product.

this leads to the RSX - the PlayStation3 GPU which is based heavily on G70. 
It is now speculated that the RSX will have 28, or even all 32 pixel 
pipelines working, whereas in the current G70, only 24 are working.

that is the point of this post - that Nvidia is holding back what their new 
GPU is capable of - which will be fully used in a new card (i.e 7800 Ultra) 
and in Sony's new console (PS3).   now I will provide the sources for this 
speculation which is really not my own personal speculation

http://www.beyond3d.com/#news24313
http://www.ga-forum.com/showthread.php?t=53384
 http://www.tinyurl.com/dfrch
http://www.beyond3d.com/forum/viewtopic.php?t=24411