Flush draws.. when? why?

Hello everyone!
This is my first post. You'll have to pardon my english. It sucks.
Well, i'd like to know when is it right to draw to a flush.
For instance, if you have 34s and the board is 2 suited... is it right with such a small cards?
When you have only one card to draw you must have one of the two top remaining cards, that's the only thing i know for sure.
But with the previous example, should i call a reraise if the pot odds are correct? Should I bet/reraise when i get my flush (assuming a non paired board)?

Thank you so much. English corrections allowed [img]/images/graemlins/heart.gif[/img]