ed what is your memory at now? Go to options>status and post that file free number here. As far as the media card, what OS do you have? To find this either go to options>about or type the word myver and hit spacebar in a text and it will give you your current OS...
I have 3785430 bytes of free space and my OS is v4.3.0.104 (platform 2.6.0.59)
Wow dude no wonder your messages are deleting that is waaay to low. You need to delete some themes, third party apps, etc. to free up some space. That link is a great one that can help you increase some file free memory. Also are you making sure your pics, ringtones, etc are being saved to your media card instead of your device memory? And lastly the OS version 4.3.0.104 will allow you to use a 8 gb media card. Here's the limits on media cards with their respective Operating Systems...
BlackBerry Device Software version Media card size limit
BlackBerry Device Software 4.2.0 Up to 2 GB
BlackBerry Device Software 4.2.1 Up to 4 GB
BlackBerry Device Software 4.2.2 Up to 4 GB
BlackBerry Device Software 4.3.0 Up to 8 GB
BlackBerry Device Software 4.5.0 Up to 8 GB
BlackBerry Device Software 4.5.0.81 and later Up to 16 GB
BlackBerry Device Software 4.6.0 and later Up to 32 GB