Will you still have management responsibility for the IT functions of the company? If so, then you need to have regular meetings with this new staff person. Major purchase requests, major network, sever, and backup changes should still be discussed with you. Disaster planning, IT budgeting, information security you should be heavily involved in.
But day to day operations you should keep your nose out of once you are comfortable that they know what they're doing. Let them do their job. At the same time, keep copies of important passwords and contents in a secure place. I'm assuming you will still have to act as this person's backup if they're not available for whatever reason, so you need to maintain a working knowledge of your systems.
I once took a job as a programmer/sys admin for a small organization of about that size. By that point in my career I had lots of experience with both and was much more comfortable with the technology than the management person whose responsibilities I was supposed to be taking on. The thing is he could never quite let go and would second guess even the smallest of changes I wanted to make. Eventually I left. So don't make that mistake. Be aware of what they're doing, but don't micromanage.