We are resellers for ESET, and use it on all of our servers, and don't notice any performance issues while it is running, have a look at my page on it here -
http://www.e-clarity.eu/anti-virusIt works differently to normal anti-virus as it looks at files behaviour and can find viruses before they show up on traditional virus databases (which it also uses)
Have a look and fill in your details at the bottom if you would like a free 30 day trial
M@