Actually it can really impact you. You should keep your avaiable memory as high as possible. I realize the link below is related to the 8100 but the same issue is impacting 8300's. You should never let your memory drop below 16mg. You check it Menu>Options>Status
Here is the link on the symptons of low memory and how to free memory on your device.
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Why is my BlackBerry losing its call logs or message logs? - BlackBerryFAQ