Out of memory is exactly what it sounds like: your program is trying to hold more data on system memory than is available. The cell array alone around (316663*400*8/1e9=1) GB of memory, plus overhead, not to mention the memory required during FFT calculation. You can run
at a breakpoint before this line to see exactly how much memory it takes up. You can also make sure other programs (especially web browsers) are not hoarding memory using your computer's system monitoring tools.
If that doesn't free-up enough memory, you need to reduce how much is being used by MATLAB at any one time. Most likely, you do not need to store all cell arrays on memory at once. So you can break up the problem into chuncks, running FFT on a section of data at a time.