That is not a TrueCrypt key. You can see that the key itself is 128 bits in length, and the number of round keys is 11, which is consistent with 128-bit AES. TrueCrypt uses 256-bit keys which, for AES, uses 15 round keys and has much larger keys at 256 bits. Whatever that key is for, it did not come from TrueCrypt. Let's assume that you did get the TrueCrypt key from memory, though, and you had on your hand the full 256-bit master key. So, what could be done with it?
Could he eventually crack the key to gain a plaintext password
TrueCrypt and the newer VeraCrypt do not use your password directly as a key. Instead, it is passed through a very slow function called PBKDF2 which converts your ASCII password into a raw, binary key. However, TrueCrypt does not even use this raw key to encrypt the volume. Instead, a single master key (itself generated randomly at the time of volume creation) is encrypted with that key, and this encrypted master key is stored on the disk. When you input your password, it is passed through PBKDF2, and the resulting key is used to decrypt the master key. This master key is what is cached in memory and which is available to an attacker who is able to retrieve memory contents.
While this does mean that an attacker cannot derive your password directly from the cached master key, it does not mean the attacker cannot attack your password in other ways. If he has access to the volume header, he can guess candidate passwords and see which one unlocks the drive. While this can be automated, the slow PBKDF2 function makes this tedious for all but the weakest passwords.
or could he use this key as a key file to open the volume?
Now that he could do. If he has access to this key, then he'll absolutely be able to access the volume and decrypt any data held within it. Of course, because he can't easily reverse the key into your plaintext password, he couldn't re-use your password on other TrueCrypt volumes or anywhere else the password was used, at least not without successfully cracking it.
If you're trying to recover data specifically from disk encryption software, a simple naïve search for encryption keys in memory might not be sufficient. This is because lots of disk encryption software, including TrueCrypt (except for ancient versions), VeraCrypt, and LUKS, uses a special mode of encryption called XTS. This encryption mode actually takes two keys, called a main key and a tweak key. In order to decrypt the disk, you would need to obtain both 256-bit keys. XTS mode works like so:
Both keys are standard 256-bit AES keys, but both are absolutely necessary to decrypt the disk. These two keys are derived from the master key which itself is stored on the disk in an encrypted form as mentioned earlier. The key derived directly from your password is not stored in memory, as it is only used briefly to decrypt the master key, and then discarded.