If you are trying to recover trashed files, stop whatever you're doing on your system immediately.
"Overwritten?" You cannot recover overwritten files from any OS. Trashed files are deleted from the directory which means the space can now be used by other files. Once the bits are overwritten, they're gone. Unless you're the CIA.
But there are dozens of file recovery tools for Macintosh, try versiontracker.com.
Do not expect miracles.
This is my standard sigfile so do not take it personally: "For crying out loud, read the freakin' manual."