I very rarely fill an 8Gb CF card (shooting RAW) and that's with a 7D and the temptation of 7fps.
Moderation is the key....look for the shot

Also, it depends what you want out of the airshow....many people simply want to record everything that happens and take lots of photos. Nothing wrong with that, but I prefer to plan shots and look for them. If the aircraft doesn't fit nicely into the frame (not just filling the frame, but composed nicely), I don't bother taking the shot. Similarly, if the aircraft is just doing a straight and level pass, take a couple of shots, but don't machine gun it

.mic (click on the sig for example photos
