the release for the 8100 doesnt have to be for Tmobile. you can use any release for the 8100. check the top of the 8100 forums for the sticky, and follow the instructions on how to upgrade my OS from Main Page - BlackBerryFAQ.com
All the media card size support is tied to the OS installed on the device. That is available on this forum and our FAQ site. Hunt for that, too, while you are out hunting.
You can use your Media Card now at the 4 or 6gb size. When and if you upgrade your 8100 to a newer OS, you can format the Media card again then for the larger capacity.