I'd automatically assume CASS-ee-ah. But then Marcia is MAR-shah, so it would stand to reason that Cassia could be CASH-ah, which makes me think of cashing a check (cash-a-check, lol). Cassia (maybe even cah-SEE-ah?) with the see-ah at the end pronounced is prettier, imo. ~Heather~
Actually, I know a girl who was born in India named Marcia. She pronounces her name mar-CEE-a. She has three sibs named Michelle, Mary and Melita. A bit too close for my tastes, but I do think Melita is pretty.