If your device talks about "low memory", it is not evaluating the sd card -- but rather means internal memory. So it doesn't help to buy a bigger sd card: what you have to do is to free up internal storage.
This is an issue coming up here frequently -- so for details please look at the solutions already present:
- Low disk storage SpaceLow disk storage Space
- More internal memory or less memory + SD cardMore internal memory or less memory + SD card
- My internal storage capacity is running low, what can I do?My internal storage capacity is running low, what can I do?
- Message Memory Full, Incoming message rejectedMessage Memory Full, Incoming message rejected
There are many more -- just follow the internal-storage tag and watch out...