until now I've driven m0n0 on a regular P3 500 MHz on regular PC hardware.
I'm using m0n0 for basic routing, DHCP, DNS and have 6 3COM NICs installed.
(These will be transferred in any case).
All the suggestions seem reasonable, except the poster originally
mentioned the need for 6 NIC's. Unless I'm mistaken, most of the cases
mentioned are for mini-itx boards.
I would personally lean towards a high-end P3 like a used Dell
GX150/GX200 with a CF card, 256MB RAM and three(3) 3COM or Intel dual
port NIC cards.
CF .. http://wiki.m0n0.ch/wikka.php?wakka=CompactFlash
NIC .. 10/100mb ..
Intel 82557- or 82559-based Fast Ethernet NICs ( fxp(4) driver)
Intel Pro/100 S Desktop, Server and Dual-Port Server Adapters
NIC .. Gigabit .. http://wiki.m0n0.ch/wikka.php?wakka=GigabitCards