-The normal system has internal memory and also an SD card, and the "new" model has internal memory and uses microSD cards. The internal is small, like 2GB or something and is filled mostly by the OS and updates. The normal XL includes a 4 GB SD card and the New 3DS XL includes a 4 GB microSD card. The systems can officially recognise SD or micro SD cards up to 32GB, but it's possible to use up to 128 if you format it properly.
-DS game cards and DSiWare are both fully playable on both the normal and New 3DS. Anything that may require an accessory that used the DS's GBA slot will of course not work (Guitar Hero for example).
-Both systems have a headphone jack, and there's a certain cable you could get to connect it to external speakers. I don't know what this cable might be called but I have one myself.
-The AC charger is MSRP $9.99. GameStop probably has them, other stores might too. Nintendo's Website certainly will.
-The only announced New 3DS exclusive is Xenoblade which will release in the US in April. A few games have functionality only when played on the New 3DS though; for example Smash Bros. can use the C-Stick for smash attacks, and Miiverse is available.
-No confirmation on Xenoblade on Wii U, but seems unlikely for now. The 3DS version isn't out yet so tough to say, but the game doesn't really add anything new so playing it on Wii would be fine. It'd also look better on Wii.
-Retail 3DS games come on game cards that are the same shape as DS game cards, which are slightly larger than normal SD cards. The only differences are that 3DS game cards are grey instead of black, and they have a little notch at the top to prevent them from being inserted into a DS or DSi. Most 3DS retail games are also available digitally from the eShop, but not all (usually older games). And there are of course some exclusive download games, like Wii U has and such.