We had this topic recently, nonetheless:
The choice is down to the B777-300ER and the A340-600 with the -600 considered to be the front-runner due to the existing A340-300 and A32X fleet. Around 10 frames will be ordered to replace the B747-200 and -300 currently owned/leased.
The A380 is currently considered to be too big although this is an option for the future, rumours last year suggested that IB could lease two from GECAS. No word about the B747-400.
On the narrowbody-side IB will standarize on the A32X family which means that the B757-200s are on their way out as far as I know.
Hope this helps.