http://gbatemp.net/topic/290097-which-flash-cart-should-i-get/
I recommend the Supercard DSTWO or the Acekard 2i. For the most part, the Acekard 2i has nearly perfect compatibility while the Supercard DSTWO has not quite as perfect compatibility with perhaps a few issues with things like Download Play. The SUpercard DSTWO does have a lot of extra features, including GBA emulation: you can play GBA games on a DSi or 3DS! Unfortunately, at the time this has been posted, the Acekard 2i has not been updated to work on the 3DS firmware 4.2, but I'm sure that they will update it eventually.
Also, this thread doesn't belong in the Pokegen section. I would use the General Gaming section or the Misc Game Mechanics section. You should see the option to move it under "Administrative" above your first post, between "Thread Tools" and "Search Thread".