For anyone that has the Sandisk 6gb card and can only get 1.68gb available on it after formatting - I've solved the problem.
Put 6gb in my Blackberry 8830, asked to format and then only had 1.68gb on card.
Reformatted in phone - same results.
Took out and formatted on PC using FAT/FAT16 and FAT32 - same results.
Tried on 3 different PCs, XP and Vista - same results.
(make sure reader is SDHC compatible - supplied Sandisk reader is)
Pulled out hair - same results.
Couldn't find a 'free' solution using just Windows tools
Used Acronis Disk Director Suite 10
to increase size of available space on disc using FAT32 (can't recall exact option)
After committing action in Acronis, card now has full capacity!!!
Everything I've read says use FAT/FAT16 - BUT FAT32 worked and also works in the 8830.
Loaded up mp3s, tv epis and movies. Not as fast as my Sandisk Ultra II 2gb card but not bad.
On a different note - if you are trying to find an adapter that will let you use your current headphones with the 8830, I use this one and it works great(not any adapter will work): Seidioonline.com - Product Search - Criteria: ADA-BB8100M35FST - Headphone Adapter