The game, currently without official title (called " Pocket Monsters Game Boy Advance Version (tentative) in the publication) will be an extension of the RPG established on the Game Boy and Game Boy Color game, featuring as many as 350 Pokemon...151 from the Red/Blue/Yellow series, 100 from the Gold/Silver/Crystal edition, and 100 brand-new Pokemon, several of which will be introduced in the next Pokemon theatrical release this summer in Japan.

The overhead game engine doesn't look to be too advanced beyond what was established on the 8-bit Game Boy and Game Boy Color...which is slightly disappointing considering Golden Sun has shown RPG gamers what the GBA's capable of in this genre. But that doesn't mean the game's not going to have cool little elements throughout the adventure; the three screenshots show off some nifty effects, including water reflections when wandering past puddles of water, shadows to enhance the illusion of depth, and footprints left in sand on the beach. Unfortunately, Famitsu didn't show images of the battle sequences in the new Pokemon game, so we'll just have to wait as the game progresses into development.
 

I got most of this info from IGN.com