Let's start with the fact that if emulation is what you want then the DS is not what you're looking for - the horsepower is very limiting and the screen resolution forces coders to either use croping or resizing. Just get a PSP or one of'em Linux-based handhelds and get on with your life.

If you're dead-set on the DS, get the DSTwo - it's the best flashcart you could get, especially with the extra CPU. You can opt for an R4i Gold, but that of course has smaller capabilities. GBA emulation is worse than native GBA support via a SLOT-2 cart, but it's acceptable on the DSTwo.