The DSTwo is special, it can update itself even if the DS won't run it, you'll have no problem with it when you get it.
- DS / DS Lite
The card will work on an original (fat) DS or a DS Lite.
- DSi / DSi XL
The card will work on a DSi or DSi XL as well.
- Game Updates
Firmware updates are what let cards play newer games, these are the life-blood of the card.
- SDHC
SDHC refers to SD cards larger than 2GB (gigabytes). If there's no SDHC support, you can only use SD cards up to 2GB.
- AP Bypass
The DSTwo has special features to ignore AP in most games, letting you play them without having to wait for a firmware update first.
- RTS
The Real-Time Save (Save State) function lets you save and reload your game progress at any moment you wish.
- In-Game Guide
You can pull up text files and images in-game, in order to check a walkthrough or map you downloaded at any time during a game.
- Slow Motion
The card lets you slow down the game speed as you wish.
- In-Game Cheats
The card lets you create and modify cheats while the game is running.
- Special Homebrew
The card has multiple pieces of special homebrew, such as it's own GBA and SNES emulators, as well as text/PDF reader and video player.
- Battery Life
Unfortunately the DSTwo's extra CPU and RAM (which give it the special features) eat into the battery life a bit.