Well I guess it is accurate since it is built into the kernel...
As for resetting it to zero, I don't think this is necessary... simply remember the last number and substract it
I will try to find out more about this... this could be very interesting.
for bandwidth on the whole server, I highly recommend hooking it to mrtg through SNMP. This will give you your bandwidth on a per-ethernet card basis, on a 5 minute average, 30 minute, ... etc.
Both upsream and downstream, with the max peak, average and current usage.
It is pretty good.