Server antivirus software

Unfortunately there is no "best" antivirus software. From a server perspective, they all pretty much perform the same and consume about the same amount of resources.  What is the server going to be doing? Exchange/SQL? File & Print? Remote Access Server? ISA Server?......?  

If it's an Exchange server, as an example, you don't want use A/V on the server - you want to use A/V that is designed for an Exchange server, like Trend Micro for Exchange or McAfee GroupShield for Exchange.

If it's just for File & Print, any one of the enterprise products from McAfee, Trend, Kaspersky, Symantec, etc. will do the job.  

Another thing to consider is if this server will ever be used as a workstation (will you ever open a web browser on it and browse the internet?) - it's not recommended, but it happesn.  If so then you shoudl also consider a malware tool like SuperAntiSpywar or MalwarBytes to periodically scan for malware.  These are free tools that unless paid for don't run in memory, but you can scan daily and get updates.

Now, what I do is I have an all of my servers directly attached to a firewall that has A/V and IDS/IPS scanning, so all traffic to/from is scanned & cleaned prior to the server seeing the traffic. This is more expensive, as these firewalls are not cheap, but it keeps my server performance at maximum.

ePO

I used McAfee/NAI ePolicy Orchestrator (ePO) for almost 8 years and recommend it every chance I get.
It is scalable (from very small to several 100,000+ host networks) and configurable (settings, scans, updates, etc) for all time zones and all functions.
My networks have typically been in the 1,500 - 2,000 host range.
On a daily basis, 100% of my computers (that have touched the network in the past 24 hours) have the most current definition files.
The detailed information (hardware, software, firmware, user info) collected from the 'agent' running on the remote host is enough to make even a crusty old Security Manager smile.
The reporting functions for my use - and that of upper management is superior.

BTW - asking Security folks which is the 'best' anything is bound to raise a ruckus.
The efficacy of several applications is about even - AT THE HOST LEVEL.
Nothing I've seen (11 years in Network Security) matches up to ePO/McAfee.
And it is configurable (repeat) for % of processor used and time of scans.

The Administrator configures all of the settings and it is taken out of the users hands - a highly desirable trait for this Administrator.

There are things you should be aware of when choosing an Antivirus package for your domain, or domain server. I wrote a little article about it that may help you.

http://www.experts-exchange.com/articles/Virus_and_Spyware/Anti-Virus/The-Best-AV-package-for-your-domain.html

BTW Symantec EP, while far from perfect, is decent IMO and to avoid the heavy load kak1004 mentions you can set the Realtime Protection to only scan on modify, not every time a file is opened.  You can also find a registry file to turn off the startup scan (which is rough on older computers) and if you get Multi-Tier Protection (a bit more) it covers Exchange Server (and amazingly good anti-spam as well.)

I have not found an AV that catches everything but SEP is not bad, have been using it on several sites for years now.