yes ChrisRay have many really Good points ..
but also i will like add ,that in games TWO video cards ,from diferent
companies that are 100% directx9 compatible , to the letter.. as you say it. can have a huge diferences in performance in games..
even if both follow Microsoft . why?
because there can be also other hardware diferences ,like
PIpelines organizations,Fillrate ,bandwidt and other hardware diferences ,
that can make an aplication or a game benefit more the design of
X company than the design of Y company..
in other words the same GAME or benchmark (3dmark)
can be written in many ways ,with exactly the same IQ and even better,
but with the performance switching in favor card#1 over Card#2 and in the other way.
it all depends of the game developer choices, efficiency.
and at the worst their integrity. good game developers will program
in a balanced way . the best IQ/performance for both cards.
Design 3dmark2003 with heavy use of multitexturing ,heavy use of NVidia PS1.x ,or 2.0+ (MS standar) and force ATI to multipass at all times,what Nvidia can do in just ONE PASS ,and you will see ATI claiming the benchmark is not reliable..