Well, there is no sede vacante yet and will not be until the pope’s abdication takes effect on 28 February. John-Paul II issued the apostolic constitution Universi Dominici Gregis which among other things dropped the traditional requirement for a 2/3+1 majority. Benedict XVI amended that constitution to restore the 2/3 rule but restrict the election to the 2 leading candidates once certain conditions are met. They are the same conditions as allowed an election by absolute majority under John-Paul’s rules. Many people argue that Cardinal Ratzinger would not have been elected without the lower majority established by Universi Dominici Gregis.
All cardinals except those over the age of 80 can vote. In theory any male Catholic is eligible. It has been some time since a non-cardinal was elected.
The media consensus seems to be that Cardinal Turkson of Ghana is the most likely candidate among the papabili but a more ancient consensus is that whoever enters the conclave a pope comes out a cardinal. There is an equally ancient consensus that fat popes are always succeeded by thin popes. Wojtyla and Ratzinger were the first time in many conclaves that 2 popes in succession came from the same faction among the cardinals.