Not sure how we'd get 1.5GB of RAM using an even number of memory devices...unless there's a third RAM chip hidden away on the reverse side of the board, that is. That would require a wider, more costly and more complicated bus and memory controller, more pins on the GPU and more traces in the board. Unlikely.If so, that's quite a change to expectations. It means split RAM, 512MB GDDR5 + 1536 MBs slow system RAM.
If there's actually a split between different RAM types I would much rather think it's 1GB of each type of memory. There could be 4 GDDR devices, with two more on the reverse side running in clamshell mode that we haven't seen.
That might also be the reason Nintendo reserved a full gig of RAM for the OS, to stop devs from whining and worrying about having to manage separate memory pools. All DDR3 would be OS-only, GDDR would be for games. <--- Hypothetic speculation <TM>