Creating a memory image is the correct method for preserving the state of volatile memory because it captures all the data contained in RAM at a particular moment, which could be lost if the system is powered down. This needs to be done before any other action to ensure that potential evidence, which may exist only in memory and could be crucial to understanding the breach, is retained. Encrypting the disk does not address the volatility of memory, changing file permissions could alter metadata and may not be effective for data in memory, and disconnecting from the network, while important, does not by itself preserve memory contents.