IF true (which I hope so!), then 2-3GB. But this is Nintendo. I hope they learnt from their mistakes...
But we've seen things like video chat, so multitasking doing 2 things with the controller and 2 things with the console means enough RAM for 4 tasks at the same time, which can be switched between 2 screens. And then you have the 3DS connectivity...
This seems plausible.







