I've been hosting codinghorror.com at CrystalTech on a dedicated server for about six months now, and I've been quite happy with the performance and availability. Any downtime has been completely my fault, and they've been responsive the few times I've requested support.
So when we needed a beefy dedicated server to host stackoverflow.com, I naturally turned to CrystalTech again.
Here are the specifications for what will eventually become the dedicated stackoverflow.com server:
Windows Server 2008 Standard Edition x64
Dual Quad-Core Xeon E5320 (1.8 GHz)
4 GB RAM
271GB SAS hard drive
It's amazing how cheap server hardware has gotten.
I'm hoping we won't run into scaling problems anytime soon with this dedicated server. As Ted Dziuba succinctly points out, scalability is usually the least of most startups' worries.