Though it looks like Chain of Memories is going to be a recap, I recall that it's been said that it's a very important part of the series and ties Kingdom Hearts and Kingdom Hearts II together. Not only that, it introduces two Unknowns as far as we know: Axel and Vixen. We know Axel is in KHII. If you don't buy the game, I suggest you find out what happened in it, as it's probably going to be important.