The Shulhan Aruch (Orah Haim 267:2) rules that one may pray Arbit before sundown on Friday afternoon, as long as it is past the point of Plag Ha'minha, and then immediately recite Kiddush. Since one has already accepted Shabbat, he may recite Kiddush at that point, even though the sun has not yet set. By the same token, many Aharonim (later Halachic authorities) rule that is perfectly acceptable to eat one's entire Shabbat meal before sundown. Since one has already accepted Shabbat, it is considered as though Shabbat has begun, and thus he may fulfill the requirement of the Shabbat meal already then, even before sunset. The Sefer Hasidim (Rabbenu Yehuda Ha'hasid of Regensburg, Germany, late 12th-early 13th century), however, in Siman 269, writes that a person should preferably ensure to eat at least one Ke'zayit of his meal after dark on Friday night. This way, he is considered to have eaten a halachic meal after the actual onset of Shabbat. The Taz (Rav David Ha'levi Segal, Poland, 1586-1667), however, disagrees. He writes (261:6; listen to audio recording for precise citation) that once a person has accepted Shabbat, we consider Shabbat to have begun in every respect, and there is thus no need to eat part of one's meal after dark. Hacham Ovadia Yosef, in Hazon Ovadia – Shabbat (vol. 2, p. 11; listen to audio recording for precise citation), rules that one should preferably follow the Sefer Hasidim's stringency and eat at least a Ke'zayit of one's meal after nightfall. This is also the view of the Mishna Berura (Rav Yisrael Meir Kagan of Radin, 1839-1933). Therefore, although it is certainly permissible to begin one's meal immediately after one prays Arbit, even before nightfall, one should try to eat a Ke'zayit after dark. Summary: When one accepts Shabbat before sundown, he may recite Kiddush and eat his meal immediately, even before the sun set, though it is preferable to ensure to eat at least one Ke'zayit of his meal after dark.