It is worth checking your file free space from time to time as low memory can result in emails, sms & call logs disaapearing.
To check your file free space go to options > status.
The numbers you've posted are strictly internal.54mb total, 7.5 free... That's not very much free space. You should look into the guides on this site regarding clearing some memory. RE: you memory card, I believe you can only use that memory for media.