Are you sure you need above 1GHz? People have different needs, but where I work our primary domain controller supports about 50 users at a time, runs Exchange 5.5 with GroupShield and Netshield antivirus software, hosts several databases, and runs Veritas BackupExec tape backup for itself and our backup domain controller. With a single P3 933, it seldom exceeds 10% CPU utilization and is usually poking along at about 3-5%.
It has not crashed since I started working there and it runs an Intel STL2 motherboard, which is based on ServerWorks ServerSetIII LE. I guess you'd have to call it "mature" technology since Intel has discontinued all the processors it was designed for (up to dual 1GHz with 133MHz FSB), but it does run stable. Intel has a 3-year warranty on it.
If you want more power, how about a SuperMicro board that supports dual Pentium3-S with 512kb L2 cache? They have ones with and without SCSI. Those CPUs are darned expensive though.