hey just to let ya know, lots of windows mobile devices near the 400 mhz processing range and still run somewhat slow
DS? one 67 mhz processor and another 33 mhz. (from wikipedia)
edit: oh and also? the DS apparently only has 4mb of ram, while devices like my sprint mogul have 64mb ram and consistently only have about 20mb free for new programs to run.
Now, what I would suggest is first to build a GBA cartridge that consists solely of (at least) 60mb of ram and likely 100mb or so of flash memory to store the windows mobile OS that is to be emulated,
Then figure a way to connect that to your homebrew emulator, and then build a custom OS for any of the number of slot-1 DS cartridges that will unload the windows mobile emu into the DS memory, at which point you insert a custom made DS cartridge that has a 300 mhz processor in it, connect that to the homebrew emulator, and finally use the custom OS menu to launch the emulator.
Now considering that all of this may take forever and a day, what I truly suggest is maybe fire up the iPhone sdk, learn your way around that, and develop ~7.5 fully featured applications for 5-10 bucks a pop and become slightly rich in the process, in the same time it would take to work out an emulator such as this!
Good luck!
P.S.: don't forget to consult this for some additional insight:
http://gbatemp.net/index.php?showtopic=96513