Some info on your exact environment would be helpful, such as what type of clients you have and what it is that you do.
There's tons of uses for an Xserve. Computational clustering, file sharing, web serving, etc. I'm assuming, if you have AD that you have Exchange to go with it. One of my favorites is backup. You can use Xserve and Xserve RAID to backup Macintosh and Windows servers and clients, as well as applications, such as Exchange. Leverage a virus free backup environment as well as exceptionally affordable, fast storage that will fit the purpose exceptionally well.
I have a whitepaper on these solutions on my site, http://www.district13computing.com/docs/Backup3.pdf , which is also republished on AFP548.com, MacEnterprise.org and a handful of other sites. Its a great way to start using Apple Server and Storage solutions, especially if you want to start exploring more purposes for it in your environment.
Really, though, more details!