The NetBoot image for a client is stored on the server. The client will start up from the previously selected NetBoot image, whether or not you hold down the N key during startup.
To select a NetBoot startup image for a Mac OS X 10.2 or later client, use Startup Disk preferences. To select a NetBoot startup image for a Mac OS 9 client, use the Startup Disk control panel version 9.2.4 or later.