In order to boot totally diskless, you'll need a boot ROM for your network card, along with a boot image that you can burn into it using an EEPROM burner. You can get boot ROM images from
Rom-O-Matic. Also, you may be able to boot without a boot ROM, if your network card supports
PXES, though I don't know very much about this.
If you can use a disk of some kind to boot, you can still boot up an OS over the network using stuff from the
Linux Terminal Server Project. It's probably the easiest way to get a network boot going on short notice.
