I think (no promises though - so maybe keep them backupped for a while in another folder just in case) you can delete all older versions, since they are only installation files anyway. The aceshigh.exe is the one you need (which gets updated with every patch, amongst other files).
More things to clean are the screenshots. They are bmps directly in the AHII folder.
You might also want to clean out your films folder every now and then.