As long as you have a Ghost.exe on the shared folder to run from, you wont need to copy the Ghost.exe to the usb. If you're trying to map to a shared folder on a 2k3 machine, default security policies may be blocking any access.
There are many ways for Ghost to establish contact between client and server. You've mentioned drive mapping, GhostCasting and peer to peer.
As Adina mentioned earlier, a network boot package enables you to connect to a server machine using the 'Peer to peer' or 'Ghostcast' options in the gui.
Using the 'Peer to Peer' option means you'd be trying to connect to another Ghost application on the server machine. Probably easiest to run Ghost32.exe in Windows on the server.
The 'GhostCast' option lets you connect to a GhostCast session running on the server machine. You can use the GhostCast Server and start a GhostCast session without using the Ghost Console.
You can go over the the sections 'Peer-to-peer connection with Ghost.exe' and 'GhostCasting image files' in the manual to get a full understanding.
Cheers,
Bruce