find files which have a higher permission than xxx

I think what you're after is

find -perm -640 ! -perm 640

i.e search files that have at least all the permissions in 640 and that don't have 640 as the permission bits. Or, in other words, amongst the files that are readable and writable by their owner and readable by the group, search files that are executable or writable by someone other than the owner or world-readable (assuming no ACLs). You may want to add -type f to restrict to regular files, or at least ! -type d -o -type d -perm 750 ! -perm 750 to allow directories to have execution permission.

If you want to match files whose permission bits, interpreted as an integer, are higher than 0o640 (which doesn't really make any sense), you're going to have to enumerate several cases. If you look at the bitwise representation, there are two ways for a number between 0 and 0o777 to be larger than 0o640: either the 0o100 bit is set in addition to the 0o600 bits, or the 0o640 bits are set. Remove the final ! -perm 640 if you want permissions 0o640 to match.

find -perm -700 -o -perm -640 ! -perm 640