I am currently using Kaspersky and it uses very less system resources.
Everything of Kaspersky is fast except Full system Scan and whenever i started a full scan it uses memory not more than 60 mb(max).

Its detection rate is also very good.
The new 2012 interface looks good but i think they should provide us the option to choose "user mode" like Bitdefender does(easy,normal,expert) ,so that everyone can choose the one according to their need.

if you don't want to pay use AVAST ,it is great.