For the last 8 or so years I am running this setup sans QoS on a Dell Optiplex GX240, P4 2.8/400 FSB/256 KB cache, 2x256MB SDRAM. It boots from a 40 GB drive that is mostly empty.
My LAN makes gazilion of connections due to uTorrent and this rig pulls just fine. The soap box routers by Linksys, SMC and DLink just quietly passed out under that load (SMC was the most robust of them).
Used a Kingston 8GB USB stick for a while, but it died after 1 year, so I stick to a HDD.
Going to migrate that to an Atom 1.6 GHz box with 2GB DDR2 as soon as low profile Intel PCIe card is delivered (this Compaq has only 1 Ethernet port).