Depends on which model of the A340 you're talking about. The newer A340-500 and A340-600 use the Rolls Royce Trent 500 series each with 53,000lbs and 56,000lbs of thrust respectively. The A340-300 uses an uprated CFM-56 engine which is the same series as used on the 737 and some A320's. The A340-300 is the version that is seriously underpowered.
I think airlines should buy the 777 over the A340. The 777 is much better.