GSC Pokemon are new? Well, to you maybe, but hearing that is kinda weird.
So, what you'll need is:
Diamond
Pearl
FireRed
Leafgreen
Emerald
Ruby
Sapphire
Pokemon XD: Gale of Darkness
Pokemon Colosseum
Pokemon Ranger
Access to Nintendo Wi-Fi events, or alternativly, a Nintendo Wi-Fi USB connector or wireless router
A Nintendo DS
Another Nintendo DS
A Gamecube-Gameboy Advance link cable
A Gameby Advance/GameBoy Advance SP
A Gamecube, alternatevly, a Wii
A Jirachi Colosseum Bonus Disk
A Celebi Colosseum Bonus Disk
An Action Replay
In other words, good luck. You'll need it.
Note: Do not actually use an Action Replay, the events for Shaymin and Arceus will eventually be unlocked, there is no need for Action Replay.
So:
You can catch or otherwise obtain most Pokemon in Diamond and Pearl.
Methods include fishing, Surfing, walking through grass or caves, talking to Mr. Backlot after obtaining the National Pokedex, using the PokeRadar, swarming, Great Marsh, dongle method, etc.
From Pikachu to Meowth, Caterpie to Rotom, Tangela to Tropius (Yes, you CAN find those in D/P), there are a lot of Pokemon in Diamond and Pearl (D/P). However, there are some only obtainable by getting other games.
Dongle method Pokemon: Import or dongle, you need Ruby, Sapphire, FireRed, LeafGreen, or Emerald, depending on which Pokemon you are trying to get. You need these Pokemon games in your DS's GBA slot.
Legendaries: Dialga in Diamond, Palkia in Pearl. Groudon in Ruby and Emerald, Kyogre in Sapphire and Emerald. Latios in Ruby (And Emerald, if you choose "blue" when your mom talks to you after beating the Champion and watching the credits) and Latias in Sapphire (And Emerald, but only if you choose "red" instead of "blue"). Raikou, Entei, and Suicune in FireRed/LeafGreen/Colosseum (Note that which one is in FireRed/LeafGreen depends on your starter). Latios, Latias, Raikou, Entei, and Suicune are running Pokemon (Except in Colosseum), but you can ask what those are somewhere else. Moltres, Zapdos, Articuno and Mewtwo come from FireRed/LeafGreen. Manaphy comes from Ranger. Regirock, Regice, and Registeel come from Ruby, Sapphire, and Emerald, and are crucial to getting Regigigas. Lugia comes from XD: Gale of Darkness. Ho-oh comes from Colosseum. Phione and Regigigas are results of importing Pokemon from other games. Crescelia, Uxie, Mesprit, Azelf, Giratina, and
Heatran are obtainable whether you have Diamond or Pearl. Arceus and Shaymin are not obtainable right now.
Darkrai, Mew, Celebi (If you don't have the Japanese Bonus Disk for Colosseum), Ho-oh (If you don't have Colosseum), Lugia (If you don't have XD), Jirachi (If you don't have the American Bonus Disk for Colosseum or European Channel), and Deoxys are only obtainable (Except if you have the abovementioned games or bonus disks) through Nintendo Events. Therefore, the only way to get them now that the events are long over is by trading.
And the starters?
Kanto starters: Bulbasaur, Charmander, and Squirtle.
Import from Fire Red and Leaf Green.
Same for Hoenn starters, Mudkip, Treecko, Torchic are imported.
Johto? After you beat Emerald and fill your Hoenn Pokedex, you get a choice of three starters from Professor Birch: Cyndaquil, Chikorita, and Totodile.
Then, if you don't want to play Emerald all over again and collect all those Pokemon, or if you don't have Emerald at all, we look at XD. Beating Mt. Battle without leaving at any time or even accesing the PC earns you a starter knowing a powerful attack. You get to choose which one again, and beat Mt. Battle again for more.
Sinnoh: Get Pearl. Trade starter that you don't already have in your Dex to Diamond using another Nintendo DS. Restart Pearl using the Factory Reset (A certain combination of buttons which I don't remember) and pick the third and last starter.
Also, some Pokemon can only be evolved by trading or trading with an item. Therefore, you need another DS or a friend with another DS. You also need this for getting all Sinoh starters, see above, or for getting Manaphy from Ranger. Sadly, a DS across the country won't help you with the last two.
Oh, and Diamond, Pearl, and another game called Battle revolution for the Wii are the only games with Wi-Fi capacibility. No using Wi-Fi with GBA.
EDIT: I took so long in typing this that several posts are in between this and the last post when I started typing.
So, there are 493. You can obtain 491 species currently, and 492 (Shaymin) and 493 (Arceus, the creator of the universe Pokemon) will be obtainable in the future.
EDIT2: Ditto is need for a few Pokemon, such as Phione.
