"You require more than mass-market consumer hardware, so first stop looking at that market filled with attractive packaging and empty promises.
I filled similar requirement using a Fabiatech FX5624 which I acquired from Mini-ITX.com. It is wholly fanless and features 2xGb Ethernet + 4x100Mb Ethernet. I run it as a VLAN switch on the trunked Gb Ethernet ports, as a richly featured border router with my ISP on one 100Mb Ethernet port and it took all the network loads I could throw at it without a hitch. If you want even more Gigabit Ethernet ports, the Fabiatech FX5625 has eight of them. I run Debian on that host and I get it to do anything I can imagine - I love networking with Debian !
But building a router from scratch with Debian may not be your cup of tea. So if you require solid performance from something that will work out of the box, take a look at products from Mikrotik, such as the RB2011L series - the OS is proprietary but that is probably what I would choose if I had to deploy a packaged solution in a hurry - I particularly like how they are transparent about the hardware's performance and the OS's functionnality."
