Don't worry, this is a frequently asked question. You'll need to wait until Fire Red and Leaf Green are released first, before you can get the new Pokedex with space for all 386 Pokemon on it.
All you have to do is trade with FR/LG, and you get the new Pokedex on your Ruby/Sapphire cart.
To trade to Ru/Sa from FR/LG you must first collect the Ruby and Sapphire Plates from the Islands, which are acessable after you defeat Blaine on Cinnibar Island.
South of Cinnibar are 9 new Islands. On Island 1, after beating the Elite 4, a cave opens (2 Rockets were previously guarding it) In there, there is a Braille Message and the Ruby Plate.
On Island 6, there is a cave. Open it by using cut and you see the Sapphire Plate. However, it gets stolen by a Rocket member and you must follow them back to Island 5. The Rocket HQ is now open and you must navigate a conveyor-belt style puzzle to get the Plate back.
With the Ruby & Sapphire Plates, you can head back to Island 1 and put them onto the big trading machine, and there you go! You can trade with Ru/Sa, and get all those old Pokemon onto your Hoenn Team.