Currently the only two options you should be looking at is MT-card or Gateway. Lok at the FAQ posted above for finding a compatible unit with <4.5 firmware.
MT-card currently is the best. You can put more than one rom in a microSD card, and so far has support for all the latest games (except Pokemon X/Y and Animal Crossing)
Gateway has the same support as MT-card, however you can only put one rom on a MicroSD card. The next update they've been promising has support for multi-rom (in a much more elegant fashion than MT-card) and support for the 3 games not yet working. However they have been promising this update for 3 months now. No one knows when it will release, or if it will.