Upgrading to the 3DS is a great idea, since the console has a myriad of great titles. But if you can keep your GBA:SP, do it too, because there's no GBA VC on the 3DS (unless you get an Ambassador one), and the GB(C) VC is lacking good games.
So, yeah, if you can, upgrade, but keep your GBA. It might be useful. I still have mine, and sometimes, I play it, and believe me, 3DS + GBA:SP is a great combo, since you play any title from the GB(C)/GBA/DS/3DS, NES and Game Gear through the VC.
So, it's like carrying four consoles in two.