Firstly, if you can go DSi, the screens look more vibrant.
Secondly, yes you should go into the fourth generation. I would pick up platinum for the extended storyline, and extended pokedex along with the battle frontier. Either Heartgold or Soulsilver is another solid choice, with both of them being remakes of the fantastic second generation.
Also, with black and white coming to DS, buying one now would make sure that you would be able to play them when they are released.
I think you should get the Nintendo DS Lite. It's just under a hundred dollars, so it's easy on the wallet when buying a console. The DSi is another route to go. However, if you plan on getting the 3DS, I would wait before buying.
As for the Pokémon games, I'd get Platinum or HeartGold/SoulSilver. Diamond and Pearl will seem really slow to you after playing through Emerald.