I always feel its best to go chronologically. You will learn to appreciate the growth of the series more and I feel each game gets a little special. The earlier titles are more difficult than the recent entries but they are also shorter. Having played P3 and P4 already... chances are none of them will be too difficult for you. X is the easiest entry and III and IV are probably the most difficult imho.

I highly recommend playing through the whole series in order. The only other advice I would give is this: If you get bored with one of the titles, don't be afraid to stop and throw a different game in. Its alright to start on IV and V before finishing I and II. There is no sense in making the games an unpleasant experience by getting stuck/frustrated/bored and insisting on finishing the game before the next. Try to go in order but don't feel bad about jumping to the next installment, hell you might even find something in the later titles that will inspire you to go back and play the games you didn't finish.