Dual proc p3/1000ghz, 1gb ram, 3x 15k rpm 18gb drives.
On the software side, it's FreeBSD v4.6.3-RELEASE, PHP v4.3.9, Apache v1.3.31, mySQL v4.0.20 and a severely modified vBulletin v3.0.1.
The one place we're golden is bandwidth. Hosted/colocated by a company called EasyNews at their datacenter in Arizona. If there's a speed problem, it's definitely not the bandwidth.
When we get up around 100+users and another 100+ guests (particularly if a lot of those guests are hyperactive spiders), depending on what everyone is doing, I see server loads around 1.75-2.50 or so.
Really going to have to move up to something around 8gb ram, perhaps even higher. We have a bit of stats functionality coming that would cripple this machine. As is, it's buckling under the boards. As a not-particularly-tech-oriented guy, I'm going to have to rely on the recommendations of others for our next machine, balanced against our ability to raise funds for it.