Short of replacing the copy/move handler with an alternative, I don't think it's possible by default in Windows 7.
Check out TeraCopy, it's one of the free copy/move handlers. It replaces the copy and move functions with its own optimized functions. It works with Windows 7, even in 64-bits.
Even with TeraCopy, you'll get a confirmation dialog for overwriting files. But you will get 1 and only 1 notification if you select (Overwrite) "All".
In my humble opinion, it's pretty dangerous to always overwrite files without asking for a confirmation. A human error arrives so quickly ... nobody can avoid copying/moving the wrong stuff at the wrong place ...