Advantages of managed switch vs unmanage

Can someone explain for me some of the major advantages of managed switch versus unmanaged switches?

[I think my network knowledge is about a 6 on a scale of 1 to 10.]
Our company, which is about 100 people, runs a network that is entirely Windows 2000 and Windows XP.

Since a number of our Dell servers have Gigabit cards, we will buy some gigabyte switches.

Is there an advantage to using managed in our situation?

We are currently working all on 1 subnet (with a 24-bit netmask) and may eventually move to 2 subnets.

Thanks!!