A slightly radical approach, but if you really want to ensure that no (unencrypted) information from those text files ends up on non-volatile memory drives, I recommend you to use a Live CD (such as Knoppix), and only from there, mount the hard disk, copy the encrypted files to a ramdisk (everywhere else will be in-memory), decrypt and edit as needed.
The main advantage of using a live disk is that by having the whole OS in-memory you are barring most of the potential leaks, such as unexpected temporary files or the OS deciding to page the text editor. You will mostly only need to:
- Ensure that the distribution you are using in the Live CD does not automatically mount a swap partition you might have in your disk.
- Copy the encrypted file itself to a different location than the mounted drive, for safely decrypting and editing. If you feel extra paranoid you could even unmount the normal disk before decrypting and remount after deleting the plantext version, but it isn't needed.