Adding Windows administration skills to your skill set will definitely increase your value in a small to medium size shop, where people wear multiple hats. But you'll also need to know virtualization and storage. The former is not that difficult, you can download a free version of VMware. Storage is trickier, since your next employer may have EMC, NetApp, IBM, Dell, etc. At least learn as much as you can about the storage you are currently using.

One thing to do is to look at your favorite job board (Monster, Dice, Indeed), and look at job postings for Windows admins, and SQL Server/Windows positions. Look at the top of the requirements list, and see if these are things that you would be able to pick up quickly.