I just threw together a system today that works great! I was going to use an RB532/A with the 6-port daughter card, but seeing as NO ONE in the USA seems to have them (AHEM! MT are you listening?!?), I was forced to use a PC.
I used an ASUS A8V Deluxe board with an AMD Socket 939 3700+ processor. It has 5 PCI slots that I completely filled with three Intel PRO/100 cards, a 3COM 3C905B-TX, and a 3COM 3C905C card. I used an ATI AGP card and 512MB memory. Don't use an AGP card with a fan, the clearance is too tight between the AGP and the first PCI slot.
To my delight, after booting to the 128MB DiskOnModule drive, I found that MT properly detected the on-board Gigabit ethernet port as well, giving me 6 routable interfaces! I wasn't expecting that, I would have been happy with 5.
I'm not sure why you asked for "no audio", IRQ conflict possibly? The A8V does have 6 channel audio, but it doesn't seem to conflict at all even with 6 ethernet interfaces running. Just to be safe, I disabled COM2 and the printer (parallel) ports on the mainboard to save a couple IRQ's.