Unlike with the Wii, Nintendo will (most likely) need to change the entire hardware architecture of their next generation system; and even if they choose to maintain the strategy of the Wii (and produce a very low cost, small and low energy consumption console) it is likely that the console would be a noticeable graphical improvement over the HD consoles.
To understand what I mean, consider that what is available as a GPU for a low cost ($500 to $750) laptop probably falls fairly close to the energy consumption and cost requirements for a console like the Wii, and the GPUs in this class are already in the performance range of the HD consoles; and in some cases they outperform the HD console’s GPUs. If you assume a Q4 2011 release, the GPUs available in this range would clearly outperform the HD consoles.
If Nintendo is willing to go for a somewhat more expensive and energy hungry chip (possibly to the extent that is possible if they moved to a laptop disc-drive to make space and used a fan for cooling) they could produce a system that is (potentially) dramatically more powerful than either the HD consoles.
