BlackBerry Device Software 4.2.2 -- Up to 4 GB
BlackBerry Device Software 4.3.0 -- Up to 8 GB
BlackBerry Device Software 4.5.0 -- Up to 8 GB
BlackBerry Device Software 4.5.0.81 and above -- Up to 16 GB
from the
FAQs
the way I read that, your carrier's version would be fine for an 8 GB card.
I only belabor the point because you seem so unsure.
instructions for upgrading here