Since I'm frugal (AKA cheap), I tried AGV, avast, and AntiVir and really couldn't see much of a performance hit worth mentioning with any of them, but I am a newbie
and my needs are modest. I only ran the resident scanner on all three. I have 1 gig, AMD 3000+, and XP SP2 and I only use Firefox.
I just had some issues with AVG and AntiVir and I have had none with avast so that's why I'm staying with it.
Just my $0.02 (USD) worth.