you can play DS games on it with physical cartridges and you can play GB and GBC games that you download from the e-shop (although they dont have very many up there yet) and you cant play GBA games yet, but the ambassadors will be able to play 10 of those later this year
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DS directly, and I think there's some gb/gbc games on the e-shop. GBA games will hit sometime before the year ends (i think), though whether that's only for the ambassador games or not remains to be seen. But DS is the only ones it can play carts of.
When your only tool's a hammer, your problems begin looking like nails.If your hammer or technique's bad, those problems may become dull or bent out of shape...