As im a fan of us company rackable and there half depth systems, check this out.

Rackable Systems 2U Servers x3

Single Opteron Dual Core 4GB Ram, 2x 160GB SAS RAID 1 running webmin, ruby project system and bind dns

Twin Opteron Dual Cores 8GB Ram, 4x 160GB SAS RAID 0+1 running cpanel/whm for application development. Running daily,weekly,monthly backup retention to s3 buckets mirrored.

Single Sempron hacked to dual core 8GB Ram, 2x 640GB Spinpoint SATA II drives in RAID 1 running windows xp (backup, terminal server etc)

Twin Opteron Dual Cores 8GB Ram, 4x 160GB SAS RAID 0+1 running rysnc to clone first cpanel/whm server incase of failure

APC Smart-UPS 750VA USB RM 2U 230V running agent to first cpanel server to shutdown on low battery, 5 mins left on battery.

Plans to add further ups or network shutdown for rack

Images to follow once im done also there is a guide here which shows you how to setup cpanel behind another server on an internal ip