Err... 3ghz tells us absolutely nothing.
The model number is actually what counts here, not the gigaherz speed of the fact its a xeon. And they range form 10 cores per processor to 1 core per processor and 20 - 1 hyperthread.
So back to the question:
how many threads should i enable in batch render
well atleast 1 per hyperthread, if you have enough memorty to keep them busy. However you may also want to allocate less if you dont happen to have that meany licences around, or you dont want to suspend the foreground processing etc.
Or you may want to render 2 simultaneous jobs IF you hev the bandwidth/memory/licences to spare
also should i enable hyperthreading in bios?