The ultimate impact and result was death. The ultimate result of death was the end of the feud between the families. The immediate result of the conversation was two youths clandestinely dishonoring their family duties causing the deaths of Tybalt and Mercutio. The result of this was that Romeo was banished and estranged from his bride leading to the resultant foolish plot to feign her death. The impact of this was that the families and the kingdom lost two of their fairest flowers.
The lovers had observed each other before they actually spoke, and the result is that each is bewitched by the other. From this point on, they only really hear each other, and the words and good counsel from those around them is only heeded when it leads directly to them being together again.
Since Romeo was on the "rebound" from Rosaline, any pretty girl would probably have caused Romeo's heart to skip a beat. However, in speaking to Juliet, a member of his family's enemy, his heart not only skips a beat but he also loses his ability to be reasonable and logical. Of course, Juliet does not know that Romeo is trying to get over another girl, so she does not know the possible genesis of Romeo's "love" for her. They both come into this relationship with different "baggage" of sorts, but since they don't really have any meaningful conversations, they do not know what the other is truly thinking. So, what starts as an innocent meeting at a party ends with disastrous results.
They both definitely knew what they were getting into by speaking to each other; their actions seem to me like a little mini-rebellion against their families. After having seen and spoken to each other, their motives no longer matter, because they are completely, as post #5 points out, infatuated with one another.
I must agree with the other posters. Once Romeo and Juliet spoke, the interest they had in each other changed and they instantly became infatuated with one another. At one point, Romeo can see that Juliet is speaking, but he cannot hear her. Juliet, while speaking, is concerned if Romeo can hear her. It seems that their love relies on each's ability to hear the other.
They are smitten, and though both of them know the implications (Juliet even warns Romeo that he'll be killed if he's found in the garden later) they still pursue the marriage. Considering the realities of their world, it's difficult to imagine it ending well, especially after Romeo kills Tybalt and Capulet promises his daughter to Paris.
Their first words reveal that they are both immediately infatuated with each other. They legitimize their "love at first sight" by confirming to each other that they both feel such a strong, immediate attraction. Had they not spoken at the Capulet's house, they might have dismissed their initial feelings.
The result of their first words is the beginning of the plot events that leads them inexorably to their deaths.
The result was that they fell in love. The result of them falling in love, of course, was their deaths. So I suppose you could say that the ultimate result of their choice to speak to one another was their deaths. Of course, there is no way they could have foreseen this when they took the fairly innocuous step of speaking to one another.