FYI - I have one of the first 8100's from T-Mobile. I am currently running OS 18.104.22.168 (downloaded from a link on BB Forums). I purchased a 8gb Micro SDHC card from Amazon a couple of months back for around $20. It works just fine and did not require formatting. There is over 7gb space available for pics, audio files, etc. I have no problem using BB MediaSync and putting a bunch of music on this card.
It is my understanding that as long as you are running OS 4.5 or higher on the 8100, you can use these larger media cards (worked for me). Also, I have never had any issue upgrading my OS when using the links provided by BB Forums.
Hope this info helps.