I have also heard good reports about NIS / NAV 2007, in that the resource usage issues of earlier versions, have all but been eradicated, and this version does run a lot leaner, however, when I tried it earlier this year, I found that it increased my boot up time by about 300%, which wasn't tolerable. Since then, I have formatted and reinstalled Windows, so I will be trying NIS again when my F-Secure license expires in May.
Not sure about updating OEM versions, DG, but it could well be cheaper to buy another OEM license, rather than renewing the existing one!