Will my studying Classical Greek prepare me to read the Septuaginta?
I’m more interested in the OT than the NT.
Yes. Hebrew, too. There are plenty of Semiticisms in the LXX.
I’m experiencing the other side of the coin: with the smattering of Ancient and Modern Greek I’ve managed to pick up recently I find that the easiest reading materials are from familiar biblical passages, such as the story of Noah and the flood. I get a lot of practice recognizing words and structures I’ve learned already and pick up many new words from the context. A beautiful web site where you can sample the Septuagint is Elpenor’s (where they also have the full Greek and English text of the New Testament):
http://www.ellopos.net/elpenor/physis/septuagint-genesis/default.asp