Well - since you have AV at all, you're already well ahead of the majority of Mac users and I think Sophos is pretty good. Its been around a long time, protects against trojan and worms as well as viruses and best of all its free.
If you are interested in a pretty low cost alternative that has more expansive covereage than Sophos (firewall, anti-spyware, works on iPods, iPhones, etc) then you might want to look at Intego VirusBarrier
Plenty of hardcore Mac users still claim the best Mac AV is no AV
Plenty of those are now wondering what critical data (credit card info, user names and passwords) they may have exposed to Russian hackers via the Flashback Trojan.