a few months ago I switched from AVG to Avast!. I originally went with AVG because at the time they were the only viable option for a non-corp user running a 64bit windows. Since the release of Vista, more and more people are rebuilding their code for 64bit, and its less of a hassle to run Xp64 and more of a joy as my primary home OS.
Back to the topic at hand . . . I noticed when I switched to Avast! the system resources tied up by my av program reduced drastically. Also of note, Avast! is always a free product for the home user, so you don't have to worry about features you may want being locked as you do in the free version of AVG.