Do you have a lot of pics, videos, etc on it? Did you format it when you put it in your phone? If not you can format it by going to options>media card>menu>format card. Hope this helps...
Will that delete everything on the memory card already?
Yes
You can move it over to the PC first and then format, return it to the card. Use mass storage mode on the card and drag and drop from windows if you dont have a card reader on the PC..
As stated in post 5 you will need to upgrade your OS to use the 8gb card unfortunately.... Here is more info
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