with the bigger cameras, and shooting to the right, i wouldn't bother going with anything less then a 16gb.
I have an 8 card on my 7D and I've maxed it out a few times by accident (and yes, i dump the cards immediately as soon as i get home.)
I find if you're not shooting to the right, an 8gb card is the bare minimum. But when shooting right, it's super easy to fill up as bringing the exposure up brings each RAW file up a few MB each.
In my 5DII I've been using a 16 and have never maxed it out, shooting to the right or not.
I just purchased another 16GB yesterday to go in my 7D because I became annoyed at my shooting getting interupted by the camera telling me I was running out of room. So, my old 8GB will take up some space in my bag as a spare.
My 7D isn't usually on burst. if this helps.