In the FAQ's from Jan. 07...
When playing a 2-player game, a player does not win if he
controls most than half of the unrazed Kingdom cities.
Instead, if a player controls 5 cities at the start of the
Upkeep phase, he immediately wins. If no player accomplishes
this, then the player with the most Influence at the
end of the game wins.
Do these 5 cities include overseas cities, or just kingdom cities? It would seem pretty easy to take over three neutral kingdom cities, then skip overseas and take over two 200 strength cities to win the game quickly.