As previously mentioned season seven is the last season. The final episode will be on 14 May (in the US) and will probably be Inauguartion Day for the new President.
They have decided to kill off Leo McGarry, following the death of John Spencer who played him, and the episode in which he dies will show on 2 April. This is five WW days before the election so I expect the election result to be known on probably 16 April.
They have consulted election experts and decided that McGarry’s name will remain on the ballot (which is correct) and if Santos wins to have him appoint a Vice-President after his inauguration which is arguably possible but not the only course of action.
Of course none of this will probably air in NZ until 2007! Grrr.