There are two versions because the whole appeal to Pokemon is that there are "exclusive" pokemon in each title, and you need both versions to complete it. This has been the same since the first duo was released (Red and Blue). They want you to find people who have the other version and trade with them to complete the collection. Some pokemon can also only evolve when traded, so you can't complete the Pokedex (an encyclopedia of all the pokemon in the game) without doing that.
It originally started as 150 (Red and Blue) but as of Diamond and Pearl there are now 493 total.
There's no advantage to picking either Diamond or Pearl, it just depends on what specific pokemon you want.
http://www.serebii.net/diamondpearl/exclusives.shtmlBefore, you had to find someone who had the other copy, but with this version, you can now go online and trade with people all over. The game is set up so that throughout the game you will SEE every pokemon, but not be able to catch them. So what you do is go online, and try and gather the ones you need by asking for the ones you want. Alternatively, you can use Friend Codes with people from GAF or Evilbore or anywhere and trade with them.
Pikachu is in the game.
When you finish the Pokedex, you are given the National Dex, which allows you to catch every single pokemon in the game and get all 493. You can also connect the GBA versions to the DS, and allow you to capture some pokemon that way.
You can find a lot of info over here:
http://www.serebii.net/If you're not into the whole collecting thing, Pokemon is a nice RPG distraction, especially since you've never touched one. You should get FireRed or LeafGreen for GBA, since that is the best in the series, imo (it's a remake of the first games red/blue). Not to mention it is cheap ($19.99 new).
