So I haven't played the Kingdom Hearts series, and I'm just finishing the first game. However, I'm thinking that maybe I could try playing through the series for the first time chronologically, as opposed to in the order they were released in. The order I'm thinking of playing the games in is:
 
Kingdom Hearts 1
Chain of Memories
358/2 Days
Kingdom Hearts II
 
I could probably get a hold of a PSP one way or another to play Birth by Sleep when that comes out, but I'm not 100% sure. Do the games like 358/2 days give you more enjoyment if you've played, say, Kingdom Hearts II, which takes place later in the actual timeline? 
 
Also, no spoilers please :O

if you play kingdom hearts 2 before 358 i guess you feel more attached to roxas.... but if you play 358 then you......aww i dont know just go for it chronologically

If I remember right, 358/2 Days has some KH2 spoilers so I say go for KH 2 first, then 358/2 Days.

I think it would be interesting to play 358/2 before KH2, go for it.

I agree you should play KH2 before playing the DS title, you will get more enjoyment out of the plot from the DS one. 
 
Though to be 100% honest it doesn't really matter the literal ending of the DS game is the start of KH2 similar to how the end of KH1 is exactly where CoM begins.

Play KH1 then KH2. Then go to KH:ReCOM and finish with 358/2 Days. ReCOM assumes you've played 2 already but it's far better than the GBA release.