We use version 9 of the older Corporate Edition, but will be changing soon over to hopefully NOD32 for our network. I was asked to look into just getting a new version of Symantec AV, but for the very reason you've described I've turned it flat down, which was only one of many the reasons.
Half of our computers are still Pentium 3's, with limited memory, 128MB to 256MB. No way can they run SEP.
As most people would say, get away from Symantec. There are far better products out there.