I love VirtualBox. Really.
I just set up a dual CPU AMD64 Opteron 270 (each processor is dual core, so I effectively have four 2 GHz CPU's) with 4 GB of RAM (and another 4 on the way) running Ubuntu 7.10 64 bit server edition.
The motherboard on this server has dual gigabit ethernet ports, and I bound them together into one network port that nearly gives me 2 Gb of bandwidth. I then bridged this network port and configured several TAP devices that I can designate to the guest machines.