My suggestions for servers

Servers must

Respond quickly to simultaneous requests from multiple users.

Be available whenever users need them.

Disk channel bandwidth is critical in most servers.

Before specifying a server that will fit your needs, define the server's function. For typical workgroup file sharing, disk speed is critical. For an application server, CPU speed is critical.

In all cases,

Add lots of RAM to cache disk sectors.
Protect the server from brownouts, blackouts, and voltage spikes.
Provide redundant hardware.
Provide a reliable data backup system with removable media.

Place your server(s) in a secure server room.

Four HP Proliant rack mounted servers with hot swap drives.
Four HP Proliant rack mounted servers with hot swap drives.
There is a slide out rack mounted laptop computer fitted between them.

Blade servers

Blade servers pack more servers into a given volume -- an entire server is housed in a 'blade', which plugs into a backplane. The blades share a common power supply(s).

They're compact, but proprietary.

IBM bladecenter 3
IBM Bladecenter, with an HS20 server partially removed.
photo:Robert Kloosterhuis
Dell rack servers
From top to bottom: Dell RAID, rackmount Poweredge server, and two blade servers

What to look for in a server

CPU

Fastest that the budget will allow, with lots of on-chip cache

Consider two or more CPUs only if the operating system supports them. Otherwise, it's a waste of money.

Disk

SCSI or Fiber Channel interface. The faster and wider, the better.

7200, 10,000, or even 15,000 RPM

Low access time

Some form of RAID (I prefer RAID-1: keep it simple!)

Hot swap drive carriers are nice. Otherwise, I'll swap drives while the server is off-line during scheduled downtime.

Considering ATA disks? Think again.

A technician replaces a hot swappable hard drive
A technician replaces a hot swappable hard drive

Memory

ECC. Lots of it. The more memory, the less often the CPU needs to access the disk - that means faster disk fetches for the users.

Power Supply

Plenty of reserve power (in terms of output Watts), capable of powering the maximum number of drives that the server enclosure can accommodate.

Redundant Hot-Swap Power Supplies

Redundant: (2 or 3 supplies). The possibility of supplying AC power to one power supply from one circuit and supplying AC power to a redundant power supply from another building circuit adds more redundancy.

Redundant supplies are nice, but not as important as redundant disks, since power supply MTBF (mean time between failure) is greater than disk and power supply failure rarely causes data loss. When budget is tight, dispense with redundant power supplies before redundant disks.

Redundant Hot-Swap Power Supplies
Redundant Hot-Swap Power Supplies

Cooling

Multiple ball-bearing fans (Sleeve bushing fans, common in cheap PCs, are junk.)

Plenty of airflow around each disk drive

Ducted airflow away from CPU(s)

Low drag internal airflow path, from bottom intake to top exhaust

Packaging

Plenty of empty drive bays to allow addition of more drives

Rackmount preferred. Minimizes footprint. Accomodates expansion with same footprint.

If rackmount, mount server chassis on slides to allow servicing

Consider an external RAID drive enclosure.

Status Monitor

A front panel LCD display of system status is helpful.

Motherboard enclosure monitoring software with thermal sensors, intrusion interlock switch(es).

Is a MIB available for SNMP monitoring, or Landesk or similar remote management? In a small office it's not a deal killer, but is desirable in larger environments.