Quoting: Ken Noddy
I have considered saying 'Jestes tak piekna' Is this a bit too much, too soon?
Get to know her first. It just doesn't sound believable since that's the first think you're gonna say to her. And that's a really obvious thing to say as well, come up with something more original if you want to impress or just use a few Polish bits to introduce yourself.
"Nazywam sie <your name>" stuff like that..
All in all talking Polish to her isn't the most important, just try to get to know her.
If you know her name, what kind of movies she like, what kinds of food & what she does in her free time then you know you're doing well :) Just don't make it sound like an S.A.T. for her !!! :-)