Witness Server for Database Mirroring

How powerfull is the witness server? I am looking into mirroring our databases with Automatic failover. I assumed I can use one of our unsed DL380 G3 server for witness server if this doesn't require much horse power.
Thanks.

The only task being performed by Witness is to form a quorum and request mirror db to restore itself and acquire the role of principal . If your witness server is not holding any data and no other application/ user is accessing it, there is not much load on the witness server. You can even use Developer or, express edition as Witness.