I have two main suggestions
1:
The Acekard 2i.
A cheap solution at just $15 standalone, this will play most of your DS games without a fault, when you install the AKAIO firmware, there's no frills, just great compatibility.
This supports SDHC and DSi systems.
2:
The Supercard DStwo.
A premium solution for $35 standalone, it has a dedicated integrated CPU, capable of doing things no other flashcart can. Asides from just playing your DS games, it is capable of delivering GBA emulation, and better potentially emulation than any other flashcart can provide.
It has a wealth of real-time functions such as real-time gamesaves, real-time guides, real-time cheats support, and more.
It also has a SNES emulator coming in the future, as well a DivX video player, and it already has eBook support.
Finally, it has SDHC support and works on DSi systems.
I suggest you buy these are GBAtemp's affiliated site,
Shoptemp, they have competitive prices and good shipping rates, their free shipping arrives at about 2 weeks for most customers.
Theraima said:
Dont really know about DSi flashcarts but I'd suggest R4i or Acekard 2i. Dunno man.
I strongly advise that you DO NOT get any R4 branded device that can work on the DSi.
The only good R4 is the
Wood R4 compatible original, but those do not work on the DSi, and do not support SDHC.