Emulators "translate" instructions meant for one system to run on another.
That's not the same thing as porting which is using a common source code then compiling it to the instruction sets of a required piece of hardware.
As for whether nintendo will do it if the 3ds is capable of doing it... It's all about how much money nintendo thinks it will make on it. First off, they would almost HAVE to allow for N64 games purchased on wii to be transferred to 3DS. Why? Because they will have a LOT of angry customers if they find out they can't transfer games they 'own' on Wii to the 3DS. That immediately will cut into their revenue (a way around that would be to charge a "transfer fee" but it will still cut into profits overall) and be a negative.
Secondly they have to balance possible net gains, with total cost of making a new emulator and putting a dedicated staff to improve compatibility (to allow for other companies to use it)
Third, they will have to compare it to the possible money loss versus remake revenue.
That said, if Nintendo can rubber stamp parts 1 and 2... part 3 won't hold them back at all. This is a company that just released a new copy of Super Mario Allstars on a wii disc DESPITE the game already being out on virtual console. It could canibalize sales somewhat, but if nintendo was going to handle this intelligently, they'd leave most of the VC ports to third parties or games they wouldn't bother remaking (of which there are many, but easy to make a quick buck off of)
As for the great "can XXX emulate YYY" debate... There is no magic number... Whoever said "8 to 10 times more powerful" of a system needed quoted BS. I remember when I had an 400mhz PC that could emulate N64 better than SNES
(God bless UltraHLE) and the difference based purely on processor speed was over 100x in the SNES case. Hell, Bsnes is the most accurate SNES emulator and it requires a heftier machine than that to play games at full speed with no frame skip.