1.5 is just a re-release of Re: Chain of Memories (which is most certainly superior to the GBA original) as well as the first overseas release of the "Kingdom Hearts Final Mix", a re-release of KH1 only sold in Japan. The DS KH game 358/2 Days is also essentially turned into a movie "spanning 2 hours and 50 minutes of HD remastered cinematics from the original game, as well as new scenes"... which will be infinitely better than that awful game. Ugh.
Even if ya don't end up liking RE:COM all-that much (the game's a bit hit 'n miss, as some people just don't care for the card system... even though it's good), odds are you'll enjoy Final Mix. Better yet, you'll be caught up 'til Kingdom Hearts 2 if ya finish both games and watch through that there KHMovie.
... Wonder how many not-so diehard KH fans will actually manage t' sit through them three hours'a cutscenes.