My daughter was JUST like this! She was speaking in full, grammatically correct (pronouns and all) sentences and had over 300 words by 17 months old. She also knew all of her letters, numbers and colors by that age (I found the paper that I wrote all of her words on dated at 17 months old). She basically hasn't stopped since. She just turned 4 and has been reading and writing many words now for at least 6 months. I have wondered what I'm supposed to do with her when she starts kindergarten because she is already so far ahead and still has a whole year of pre-school to go before she is old enough for kinder, so I can feel your pain about not knowing what to do. Her pediatrician when she was that age kept telling me I was going to have to find a "special school" for her because she was so advanced. She just had her 4 year checkup and they did some testing (she has a different pedi now) and they also told us that she is pretty well off the charts. I'm not sure where that leaves us except that we just encourage her to continue to learn without being pushy. She is a perfectionist, especially when it comes to her writing, so when she messes something up she starts getting upset. I tell her that most kids her age don't even know how to write and that we do it for fun and that if she isn't having fun with it, then we're going to put the stuff away for awhile until she is ready to try again without getting upset. The last thing I want to do is burn her out now and have her hate it by the time she has to do it in school, but she really likes writing, so I want to encourage her to get better.
I'm not really sure what kind of information you're looking for....I know you said you don't even really know. But, I just wanted to let you know that I totally understand where you're coming from. My answer to you would be to continue working with her and just let her live up to whatever potential is there. I keep waiting for my daughter to even out with the other kids and so far that hasn't happened, but it doesn't mean it won't. Now that my daughter is older, I buy her the kindergarten workbooks and she loves doing those. They help her practice writing and have a lot of other critical thinking skills in them that she can practice. We're taking it one step at a time and we'll probably wait until next year to figure out if I am going to home school her for kinder and test her into first grade a year early (which makes me uncomfortable since she would be THE YOUNGEST kid in the class) or if we're going to just let her stay on pace with the kids her age and watch for boredom.
By the way, it is kind of interesting now because I have a son that is 22 months old and he is just now starting to string 2 words together. It is such a different experience between the two of them and it has been hard for hubby and I not to compare even our own kids! My son is perfectly normal and probably actually talks a little better than normal for his age, but he is way behind where our daughter was at the same age. Again, not a bad thing, just interesting. We encourage him to talk just as much and he sure is getting the hang of it!
Just try to remember that they are all different. As for what to do with your daughter...just keep her talking! If she's anything like my daughter, they are just like little sponges and are soaking up anything and everything you tell them. It really is fun, but challenging to have a "gifted" kid (I hesitate to use that word as well, for don't really know what else to call it at this point). Just wait until the logic and critical thinking kicks in at around 2 1/2. You won't believe some of the things your kid will say! Just enjoy the ride! :-)