Just wondering, is it possible to catch all the pokémons if one owns and completes both HG and SS or you're actually forced to trade for some, and/or get them from earlier pokémon games?

you will have to trade in order to get all of them.

No, you're gonna need a bunch of games. You need either Fire Red or Leaf Green, Emerald, Diamond or Pearl, and Platinum. And that's the bare minimum, and I still think you might be a few short.

You need to trade to get some to evolve I think, and I'm not sure all the RBY pokemon appear in Gold/Silver.

You'd need to trade to get 'em all. I have at least one of all the generations, so I could PROBABLY get all 493. That's if I had about 400 lives to waste on it.

Yikes.. but I suspected as much. Shifty Nintendo, stroking my collector-gene like that. Grrr. 
 
Thanks for the answers!

you can potentially obtain all 493 if you own the following games:
 
Heart Gold 
Emerald
Fire Red
Platinum
 
you'll still need to trade out using the global auction feature, you'll have to go through all four games, and you'll need one or two pokemon that are exclusive to Ruby, but that aught to cover it.

In the alst game, if you completed that Pokedex you got the National Pokedex. That's right.

Yeah you need FireRed/LeafGreen, a DS with a GBA slot, Diamond/Pearl/Platinum and HG/SS. Plus a bunch of cheats for the exclusive pokemon that you can only get at nintendo events (like Arceus, who hasn't even been given out, to my knowledge). Realistically catching them all is tough, but HG/SS DOES give you a lot of pokemon, most of the significant legendaries are there as well as a lot of older pokemon that haven't appeared in other games for a while. 

Isn't Celebi impossible to get legitly, without going to a Nintendo Hosted Event? 
 
@XII_Sniper:  They were giving Arceus out at Toys 'R' Us in November 2009.  My girlfriend and I each got one :) 

To get them all (legitimately), you'll probably need all version of the games, a trading buddy or another GBA and/or DS, and to go to special out-of-game events in order to get a specific item with will enable you to catch a specific legendary Pokemon.
 
And with that I say, "Fuck it", and just try to train all of my monsters to level 100.

" you can potentially obtain all 493 if you own the following games:  Heart Gold  Emerald Fire Red Platinum  you'll still need to trade out using the global auction feature, you'll have to go through all four games, and you'll need one or two pokemon that are exclusive to Ruby, but that aught to cover it. "

" Isn't Celebi impossible to get legitly, without going to a Nintendo Hosted Event? "

" Yeah you need FireRed/LeafGreen, a DS with a GBA slot, Diamond/Pearl/Platinum and HG/SS. Plus a bunch of cheats for the exclusive pokemon that you can only get at nintendo events (like Arceus, who hasn't even been given out, to my knowledge). Realistically catching them all is tough, but HG/SS DOES give you a lot of pokemon, most of the significant legendaries are there as well as a lot of older pokemon that haven't appeared in other games for a while.  "

Why collect all 493 when all you need is six Charizards! RAWR!

" @Chirag4:  You get absolutely nothing for collecting all of them, so while I'm not going to defend the stupid money-making scheme to get you to buy multiple versions of the game, it still isn't as big of a deal as people make it sound. You get Pokemon appropriate to the game you bought, and you'll never have need of different Pokemon to beat the game. If you're that crazy about the series,  you'll probably already own more than one. "

to be honest' I've never played Pokemons to get all of them. I was always concentrating on main PKMN and 5 more (they usually were legendary). It's a good idea to play this time to CATCH 'EM ALL!

I don't think you can get Mew, Celebii, Uh... whatever some of the other event-only pokemon are out there. Do you still need the Pokemon Colosseum Bonus Disc for Gamecube to get Jarachi? I've got that.

You would have to trade to get specific Pokemon, starters, and to evolve certain Pokemon like Kadabra.

" @TooWalrus:   GameStop recently had an event to get Jirachi.  You could use that bonus disc if you want to get Jirachi as well.  My friend has that disc.  I remember the day he bought Colosseum.  Good stuff.   :P "

" @TooWalrus:   I'm pretty sure you can use it with your Wii.  Most of the Gamecube accessories work with Wii. "