Stay away from the "big" companies, as much as they generally work, I have had so many headaches with Symantec and Mcafee.
I personally found Nod32 to be the best at speed and memory however the latest version is not as good as it used to be - I think they are now going down the Symantec trap and spending more on marketing than product development.... It is still better than a number out there, however, I just hope I can say that in 2 years time
If you can not wait, the ones I have found to be best (in no order)
Kaspersky - (again, going down hill)
AVG - (with link scanner disabled - memory/speed hog)
If you currently have no antivirus and can wait, I would take a look at Microsoft "Morro", (or try to find the beta) I have been using it for the past couple of months and have been VERY impressed. It uses next to no memory (even less than nod32), and is fast... As far as I know, it has no central management, but it is a very good bit of kit.