G6 is great for GBA compat. So far, I've only noticed 2 games that haven't worked on my g6, that I've tried. Those are Pokemon Mystery Dungeon Red Rescue Team (but I got blue rescue team so it's all good there), and Kingdom Hearts Chain of Memories (Only chain of memories I am disappointed that it doesn't work.)
Everything else I've tried works well on GBA.
If you're using the g6 only for gba, you can fit a ton of games on it. If you're using it for both, you can fit quite a few. Right now, I got 9 NDS games and 6 gba on my g6 lite.
Also the 8Gbit cart (1GB) is coming soon. Make sure you do not get the pro edition of it, which will not have GBA support.