Key length is an important measure of the strength of cryptography, but the algorithm is ultimately the determining factor. A prime example is the WEP key, which can be cracked in several minutes using a 128-bit key. DES only uses a 56-bit key, but you must test every possible key because there aren't any shortcuts in the algorithm. DES is far more secure than WEP, even though the key length is shorter.
But, given that the algorithm that you are using has not been broken, then 256-bit encryption would be beyond the scope of any present day eavesdropping.
pansophic