Kingdoms Hearts is amazing. Kingdom Hearts 2 is even better. I never played Chain of Memories as it was another medium, however I did hear they made it available for PS2.
I really don't see any graphical improvement, and I can't really vouch for Re: Chain of Memories, but if you want a good game to play, at the very least KH1 and 2 are very fun, and if you are a Disney fan, you will most likely catch quite a few recognisable faces throughout the game (Jack Skellington was my personal favourite).
The one thing I disagreed with in regards to this new edition was the trophies, they really weren't required and I could argue for days why they should never be regarded as part of the completion of any of the PS2 classics, but they can sometimes add a little something extra for if you are bored post-game.