Just for the record, you could even run Whonix completely on RAM by using a ram disk software. For Windows, you could also set the path of TEMP or pagefile* or other things to a ram disk so to minimise traces further.
But it would require a high amount of RAM.
You could even create a ram disk first, then encrypt and mount it with Truecrypt and then install VirtualBox and Whonix inside this encrypted ram disk This way even the content on RAM disk would be encrypted
*Edit: On Windows, some (old) applications need a pagefile so I didn't recommend to disable it. Set the pagefile to be on a ram disk, a small size (like <100 MB or 1 MB) would be enough, assuming you have a high RAM.