Network boot from a non-default NIC on a HP ProLiant G7 server

On G7 and earlier servers, this can be changed in the BIOS. This assumes onboard interfaces.

(I hope you have ILO access)

Press F9 during POST to get into the system BIOS menu (ROM-Based Setup Utility).

Follow the menus from System Options > Embedded NICs > NIC X Boot Options.
Select the NIC you wish to boot from, and flip the Network Boot flag.

You'll need a power cycle to make that change effective.

Edit:
This is also available from the host OS (assuming the server is already running and has one).

[root@Tantalalicious ~]# hpasmcli 
HP management CLI for Linux (v2.0)
Copyright 2008 Hewlett-Packard Development Group, L.P.

hpasmcli> help enable pxe
USAGE: ENABLE PXE [ NIC1 | NIC2 | NIC3 | NIC4 ]: Enables PXE support on the embedded NICs.

There's also a way to set this from the RBSU (ROM-Based Setup Utility):

    SET CONFIG NIC 1 BOOT OPTIONS DISABLED
    SET CONFIG NIC 2 BOOT OPTIONS NETWORK BOOT

For other options, using

    SHOW CONFIG SCRIPT

is super-useful as a reference.