Good luck, but you have a very tough problem ahead of you. Consider the situation where the memory page containing the plain text is swapped to disk. The disk block to which the memory page was written could lay unchanged for a while. Even worse, it could be claimed by a file which only uses the start of the block, preserving all or a part of the plaintext at least until the file is deleted.

Bruce Schneier touched on this in recent blog entry and Wired article Choosing Secure Passwords. (Look near the bottom for "Forensic Toolkit".)