Like all Xserves, the Harpertown model is self-aware to a fault (pun intended). It's loaded with thermal, fan rotation, current, and voltage sensors. Xserve reacts to trouble on its own, and it shares the statistics from these sensors with you in detail. That, in my opinion, is one of Xserve's best features. But sensor and system configuration data are also available out of band during operation and when the server is not running. Every Xserve ships with a baseboard management controller (BMC) of Apple's design that conforms to the IPMI 2.0 standard. Access to the BMC is protected by redundancy, with both of Xserve's onboard gigabit Ethernet ports wired into it. Apple's GUI-based Server Monitor taps the BMC from any Mac through IPMI, and from any POSIX-based system using an open source command line tool.
OS X Leopard supported hypervisor-based virtualization before Windows Server did. Despite the name, Parallels Desktop runs multiple instances of any mix of 32-bit x86 OSes, including most Windows Server versions, along with commercial and free Linux distributions, BSD variants, Solaris, and more esoteric choices. VMware Fusion does the same, with support for 64-bit operating systems as well. Both Parallels and VMware are working on server-specific editions of their Mac products; Parallels Server for Mac is in its fourth round of beta testing now, and has just taken on the ability to host OS X Leopard Server as a virtualized guest. Only Leopard Server can host Leopard Server, but this capability will soon make Xserve the only box that can run all commercial x86 operating systems in virtualization.
Stripped or loaded?
The least you can pay for Xserve is US$2,999 for a four-core machine with 2GB of RAM. This is capable enough for small businesses, but the empty CPU socket can't officially be filled post-purchase. Xserve's sweet spot for pricing is US$3,999, which buys you an eight-core machine with 2.8GHz CPUs and 4GB of RAM. Incremental improvements to the base config start to get pricey, which is a reflection of component costs rather than Apple margineering. For example, Xserve with 8GB of RAM and two 3.0GHz quad-core CPUs prices out at US$5,799. You know that RAM and hard drives will get less expensive over time, and you can upgrade both of these later. As for paying extra for higher CPU clock speed, you would need a stopwatch to measure any difference in application performance from that extra 200MHz per CPU.
The pleasant surprise is the price of a fully loaded Xserve. Harpertown Xserve with eight 3GHz cores, 3TB of internal hardware RAID storage, and 32GB of RAM cruises in at under US$10,000. There are 1U x86 rack servers with smaller price tags, to be sure, but none that can be taken so far in one chassis as Xserve for the money, and no PC server carries pervasive big iron design to the mainstream as Xserve does.