Without actual statistics and information it's hard to say.
But just guessing, since most sites are not nearly as popular as the site maintainers think or want them to be

That 200GB of transfer per month is going to be just fine.
I doubt you're going to have 10,000 simultaneous visitors or even 50 visitors per day to start with.

Start small and see where things go.
Say your page was 100KB in size (probably a large page for a "normal" site and possibly a smaller page for a flashy/media site). At 200GB per month, your page could be downloaded 2,097,152 times in that month. (if my math is correct). That's nearly 70,000 page views per day.
Think big, sure, but start small. You can't get to the top of the stairs in a single step.
