I've sat back for long enough reading this stuff, a comment is in order.
1) The link in question shows the r520 has a 512bit controller, but as of the moment it will have 256bit memory.
2) Perm: you are comparing vectoring (altivec in this case for the g3) with scalar. That's a no no if you want to compare performance. Certain things work quite will in vectoring, others don't. However the g3 itself is not a 128bit processor, just as a pentium4 is not a 128bit processor (though sse2 is). In the same sense the gpu is not 512bit because the memory controller is. If you want to compare altivec to sse or sse2, well that's another story.
3) x64 is a headache, and the only real benefit that has come out of it is the extra registers that were not there before. Yes there are some programs that can take advantage of it, but the vast majority have not (and probably will not).
4) Last gen: yes the n64 ran out of ram and that was a bonehead move. I honestly don't think we will see a repeat.
5) If you really want to get down to how GPUs work the 512 bit controller talk is silly unless there is a 512 bit ram to go with it. As that would require a lot of chips and a lot of layers (read: money) I don't think it will be too much of an issue. If it appears, great ATI just opened up an enormous amount of memory bandwith. If not, it's a gimmicky feature that has potential.
edit: (see below) well in that case I have no idea what perm was talkign about.