ATI have commented in the past about the spread of cards released and that as time goes on there would be more steps from low end to high end. So going for 512 bit at the high end doesn't mean that low end won't be 64 bit but rather that now you can have 64,128,256,512 bit cards. This means it should be easier to have a price point for all pockets IMHO.
True enough, but I think there's another aspect that comes into play.
ATi's die sizes have been pretty huge in the past for what they were capable of vs the competition. I have to believe a larger bus takes up a bit of space. I think RV630, for example, is 128-bit to keep it cheap as possible to produce, even at 65nm, even at the cost of some performance it may or may not need. If the <80mm2 rumor is true, and it competes with the 8600 series which is also supposedly 128-bit, no doubt that chip is a huge winner for ATi, as mainstream is where the $ are. The 8600 series chip could possibly be twice as big...Putting them in the situation Nvidia was last gen with the ability to chop prices if need-be, while also being able to create 'x2'-style cards if need-be to compete.
That brings me to my next question...When will we see a 256-bit R600 part if that is indeed the sweet-spot? I always thought parts would follow the R5xx scheme with Rv630 being 1/4 R600, and RV660/670 would follow and be 2xRv630 (with the same bus) and 3/4 R600 respectfully, but I also expected Rv630 to have a 256-bit bus...So I dunno. I guess Rv660 is still the best candidate though...perhaps 256-bit and 1/2 R600...but where does that leave R670? 512-bit? 256-bit? 384-bit?