A reader wanted to know who was the first U.S. President elected by the 50 states. This was John F. Kennedy in 1960.
However, if the question was "Who was the first President to be elected by the current size (538 votes) of the Electoral College?" then the answer would Lyndon B. Johnson in 1964. The District of Columbia (D.C.) was awarded three electoral college votes in time for the 1964 presidential election, despite the fact that the U.S. federal capital does not have any U.S. Senators or Representatives (there is a non-voting "Territorial Delegate" to the House of Representatives).
The number of Electoral College votes per state is determined by 1) the number of U.S. Senators (two in each state), and 2) the number of House of Representatives members (minimum of one, but otherwise in proportion to the voting population).
If the question were: "Who was the first President elected by the current distribution of electoral college votes?" then the correct answer would be George W. Bush... in 2004 (not 2000). Because the number of Representatives is re-calculated every ten years after the census, and the last census was in 2000, the 2004 was the first time that the current electoral college distribution was used, and 2008 will be the only other time.
Finally, some thoughts to conspiracy theorists looking for "proof" that the census of 2000 fixed matters the Republicans' way.
1) The most recent census was carried out during Bill Clinton's Democrat presidency.
2) The last congressional seat to be allocated was given to North Carolina, a decison that was taken to the Supreme Court by the state of Utah. Both states went Republican in both 2000 and 2004.
3) The Supreme Court ruled that North Carolina should get the final seat, not Utah. For the record, the Democrats did not bother campaigning in Utah in 2004, although North Carolina was at one point considered a long-shot for them.
So the evidence does not support a fix by Republicans in the allocation of Electoral College votes.