My hair is extremely blonde already, however has a yellow tint to it. I want a few white highlights in it to break up the gold. Should I use bleach with 30 or 40 vol. developer?

In spite of claims to the contrary, the strength of the developer is a minor factor in bleaching hair. The bleach, itself is what does the lightening. The developer only provides a vehicle to dissolve the bleach (bleaches are all solids) and move it into the hair shaft. Stronger developer may increase the speed of lightening, but it will not affect the ultimate lightness that is achievable.

Also, the stronger the developer, the more damage accrues to the hair. I always recommend either 20- or 30-volume (6 or 9% by weight) developer and never use 40-volume developer with bleach (if you want to preserve the "health" of your hair).