I've only ever seen Zara, and I pronounce that just like I pronounce Sara and Tara but with a different initial consonant. Never seen Zarah, though.

And Sam, I think you were the only one who said it differently to me. xD I think Kent described the phonetics of it best when he said it's said similar to how people say 'carer', but then some of you guys probably say that differently too... we gotta vocaroo all this. >=]