If you want a very light but still powerful antivirus I advise NOD32. But thats very light and I can see your quad can handle that like its nothing. But if you want what I believe is the best anti-virus is Kaspersky.
Avg is good as free gets. But if you want something that works well and actually catches viruses in real time you would look no farther then nod32. www.eset.com it is bar none the best av out there. http://www.av-comparatives.org/seiten/ergebnisse_2007_1...
Check there it scored the highest out of all av's. It is definately the best.
I forgot to add is uses less resources then kaspersky and avg.
nod32 is good. I am currently using a trial version of it on my pc and it doesn't drain my system resources like norton did.But I hear that norton have got a new version out now that's also light on system resources.
My Trend Micro Internet Security 2007 virus definitions just expired. Since I had a hassel with it working properly on Vista and it was a resource HOG, I considered checking another brand out. I did some research and learned Kaspersky is supposed to be frugle on resource usage and protect as well or better than other brands. I just purchased Kaspersky Internet Security 7.0, and hopefully it will perform well.