GT Advance Championship Racing Cheats

Complete the final race in the professional class to unlock a seat which boosts all three stats.

To gain access to the Extra 2 option on the main menu, you must complete all classes in Championship mode (Beginner's, Medium, High Speed, and Professional). You must have Gold Cups on all tracks to open up this option, too.

To gain access to the Extra 1 option on the main menu, you must first complete the Beginner's, Medium, and High Speed Classes in Championship mode. I'm not sure exactly what the requirements, cup-wise (gold, silver, bronze), are though; I had all Golds on all tracks when I opened up this options.