Congratulations! I know you're in a bit of shock right now, but you'll adjust eventually (hopefully...hehe). My current pregnancy was a complete surprise. I can relate a bit with you.
The age spacing isn't too bad. Your 17 month old will be around 25-26 months old? He/she will be nearing the age of being more able to be more obedient:-) so that's a plus!
For the terrible twos, I'd HIGHLY suggest reading this book called:
"Positive Discipline for Preschoolers: For Their Early Years--Raising Children Who are Responsible, Respectful, and Resourceful"
http://www.amazon.com/Positive-Discipline-Preschoolers-Ye...
That's a nice long name, eh?! But if you can get read up on a good book like this, you will have the skills you need when you need them to help your situation not get overwhelming for you. It's a much different approach than the "Super Nanny" punishment methods. And, this method (from the book) has been proven over time to have excellent long-term results. You respect your child as you are loving and firm in disciplining and helping them be who they are and be a respectful person who feels good about themselves (whereas more punishment type of parenting often destroy self esteem and cause lots of issues later on).
Anyway, I won't ramble forever. Just HIGHLY recommend this book and her other one called "Positive Discipline" which is good for once they are a bit older. Both are by Jane Nelsen. Educating yourself so you have the skills when you need them will be a life saver for you.
Oh, and another good book "Siblings Without Rivalry". This is an awesome book too.
http://www.amazon.com/Siblings-Without-Rivalry-Children-T...
If it helps any, my 2nd baby was 19 months old when baby #3 arrived. Baby #3 was 22 months old when baby #4 arrived. Baby #4 will be 15 months old when baby #5 arrives (our surprise baby!). It is a bit of a challenge for sure, but it's more doable than it seemed like it would be. And you adjust and it gets fun living in a world of chaos where your life goal becomes figuring out how to shower every day:-)
Good luck!