If you're willing to wait, you may want to look into getting a Nintendo DS upon it's winter release this year (estimated by Nintendo). The Nintendo DS is backwards compatible with GBA games. This would be an efficient option.
Coliseum isn't all that great unless you have a GBA game, as well. The RPG mode wasn't very long compared to the GBA games. Or you could get a GCN with a GameBoy Player.
I would wait for the DS to be released.