Scientifically Insecure

I've been asked to give a talk at a conference I'm attending next month! I had originally intended to present a poster, and I was feeling pretty good about it because this is a small meeting, organized by graduate students and mostly for graduate students (as opposed to a large conference where the leaders in my field will be conducting day-long symposia or something). In fact, this low intimidation factor was one of the main reasons I jumped on this meeting in the first place... well, that, and... it's in Switzerland. w00t!!!!
So a few days after submitting my abstract, I got an email from one of the organizers asking "Your particular field is rather under-represented, would you mind very much giving an oral presentation instead?" To which I initially reflex-vomited, then asked my adviser if he thought I was capable of such things. He said yes, I emailed back yes, and here we are.
I'm feeling unusually confident about the whole thing, but I'm sure that will fade quickly when I actually start putting a talk together and imagining myself standing there, with people assuming I'm some sort of expert on ANYTHING EVER. And here lies one of This Scientist's most common problems: insecurity. I'm starting my sixth year in graduate school (and my last, God willing) and I still hesitate to call myself a molecular biologist, an endocrinologist, a neuroscientist, any of the things I'm supposed to be at this point... when people ask what I do, I say "I'm in graduate school". At what point do I go from being student to scientist?
Confidence is a funny thing. It seems like I have two modes when it comes to being "a scientist", when people ask me science-y questions: 1) total spaz that babbles incoherently and answers with as little commitment as possible, or 2) calm, collected grown-up with enough maturity to stand still for ten seconds in silence while I think it over. I've only recently began to blossom from #1 to #2 and, more often than not, #1 takes over and I end up totally embarrassing myself. The first time I gave a talk (my alma mater invited me back to speak about my current graduate school adventures) I gave the most absurd answer to a question I have ever given in my life, so absurd I will not repeat it... because I was afraid of silence, afraid if I didn't say SOMETHING they would assume I knew NOTHING.
People say "It's okay to say you don't know", but what they really mean is "Learn how to say you don't know without sounding like an idiot". I'm still terrible at the latter, but quite good at the former! 
All that to say that I'm excited, and honored. Yes, I know it's just a student-organized conference, and they only asked me because my field is under-represented, and the fact that my boss is one of the invited speakers probably gives me some credit-by-proxy even if I turn out a total sham... but MY take on it is that if I was truly a complete moron, a quick Google of my name would shout out DON'T ASK HER TO SPEAK! SHE WILL HAVE SLIDES OF RAINBOWS AND CALL THEM DATA! SHE WILL SHOW UP IN A TUTU AND IT WILL BE AWKWARD! But that didn't happen! So maybe I'm not a complete moron, because they DID ask me!
Hopefully because I'm one of the few from my particular field, I actually will know more about what I'm doing than anyone else. When I'm attending a talk, I generally give the speaker the benefit of the doubt that he or she knows way more about their talk than I do. The idea that the audience will assume the same about me is nerve-wracking, but I know I have to get over it. Screw your balls on and go for it, right? Prepare as much as possible, hope it's enough, stay calm and carry on. Here we go.