Big GPUs are set to die - First R700 info, and it is cool
 

THE ATI R600 will represent the last of it's breed, the monolithic GPU. It will be replaced by a cluster of smaller GPUs with the R700 generation. This is the biggest change in the paradigm since 3Dfx came out with SLI, oh so many years ago.

Basically, if you look at the architecture of any modern GPU, R5xx/6xx or G80, it comprises pretty modular units connected by a big interconnect. Imagine if the interconnect was more distributed like say an Opteron and HT, you could have four small chips instead of one big one.