4 NICs or 4-port NIC?

For that I am planning to use an old Pentium 3 machine.

Since you are using an older machine, I suspect I would go with whatever is inexpensive. Does your computer have a 64 bit PCI-X slot? Most of the 4 port PCI interfaces I have seen prefer one?

Since you are already choosing to use an older machine, I am not sure you will really see much difference either way.

For example right now:

• $401.99 - Intel PWLA8494GTBLK 10/100/1000Mbps PCI-X PRO/1000 GT Quad Port Adapter
• $187.99 - Intel PWLA8492MT 10/100/1000Mbps PCI PRO/1000 MT Dual Port Adapter
• $33.99 - Intel PWLA8391GT 10/100/1000Mbps PCI PRO/1000 Adapter

This means if you get 4 Desktop adapters you are playing $34 per port. If you get two 2port adapters your paying $94 per port For one 4port adapter your paying $100 per port. If it is for an older P3 I don't really thing you'll see a difference between a server adapter and a good desktop adapter. Plus with if you go the inexpensive route you can afford to pick up a spare or two for growth or replacement if something fails.

I think probably the biggest issue you need to look at is simply how many expansion slots and what type are available on the motherboard.

I'd go with two 2-port nics. They're generally easier to find, and you can get two for less than the price of a single 4-port. It also offers you more hardware redundancy in the rare even that one of them fails. For bonus points, get them from different vendors and avoid potential driver bugs as well.

Of course, all of this may be unnecessary if you just use VLANs. A lot of the router setups I've done are "one-armed routers", with a single cable coming out and all networks going through that port as tagged VLANs.

Personally I would go with a 4-port NIC.

Not a huge difference at the end of the day just less hassle.