imo it does matter, ill make an example.
a 9800GX2 is 2x16 ROPS, 2x128 sp's and 2x512mb ram over 2x256-bit bus while one gpu can only access one memory, and all this is linked by a scalable link interface.
so the question is, will it perform as well as a single gpu that natively has 32 ROPS, 256 sp's, and 1024mb of ram over a 512 bit bus?
imo the answer is a definite no, for scaling reasons and others, like a core 2 duo at 3ghz doesnt exactly mean you have a 6ghz computer.
so yes the 9800GX2 does possess a total 512 bit memory interface, and a total of 1024 mb ram, but for reasons i went in to, its not really a true 512bit, yada yada yada.
sorry to bring it back up again, but i think its an interesting subject guys