Lots could be said here, but probably the most helpful tip for you would be to buy a bike from a local shop where you can test ride and get help adjusting if you need it.

Sizing a bike isn't that hard for most riders' needs, but without having a basic understanding of the numbers at issue, you're making it too hard on yourself, and I'd say, getting ahead of things.

But, if you're insistent on continuing with the Bikesdirect thing, I'd suggest you follow the vendor's sizing guide, not some joe-schmoe on the internet, like me, who may or may not know what they're talking about. A second tip is that erring on the small side is much better than erring on the too large.

Good luck.