 
 
   It depends on the attack.  Adding a bit to the key doubles the amount of
work to be done in a straight brute-force attack, where you try every single
possible key until one works.  Processing and storage requirements for this
kind of attack on a 128-bit key seem like they ought to make it effectively
impossible.  However, there may be other attacks whose difficulty is (for
example) proportional to, say, 2**sqrt(n), or some such.  Also, a long
key does you little good if there is a way to incrementally guess a little
of the key at a time....