Parshat Toldot actually begins "ואלה תולדות יצחק". I have heard that the reason people call the earlier parsha "noach" is because the entire parsha deals with the story of Noach (and the listing of his descendants). "Toldot" on the other hand, although it begins with the story of Yitzchak's life, quickly switches to the story of Yaakov and Eisav as brothers. Therefore, instead of calling the parsha "Yitzchak", we call it "toldot".

I have heard that the reason people call the earlier parsha "noach" is because the entire parsha deals with the story of Noach (and the listing of his descendants). "Toldot" on the other hand, although it begins with the story of Yitzchak's life, quickly switches to the story of Yaakov and Eisav as brothers. Therefore, instead of calling the parsha "Yitzchak", we call it "toldot".