I have used a 256Mb card in the 3300 for over a year with few problems, but since changing to the 6230 have had problems with filling the card - Phone Full messages, although MMC still had c. 50Mb free.
I have tracked down the problem (which incidentally would also be a problem on the 3300). There is a technical limit of 512 files in the Root folder of the MMC (FAT restriction) and if long file names are used, multiple file names are used up. I was getting a limit of about 80 tracks as the filenames contain track information. I had used the root folder because files are not found by the All Tracks playlist unless they are in the root (on the 3300 they are found in the Music folder).
To overcome this I now copy music files into a Music folder on the MMC by creating a Playlist in Audio Manager and copying the playlist (not individual files). The music files are pointed to the Music folder in Audio Manager. Now I am able to fill up the card (should also work for > 256Mb cards).
I still have a couple of problems -
1) Track information (artist and track name) are not displayed on the 6230 - the same files do display the information on the 3300.
2) There is still a limit of 99 files in Music Player (All Tracks or individual playlists) - why can't the phone Music Player be changed to support 3 digits!.
3) All Tracks cannot find files in Music folder on MMC, although the 3300 can.