Good day, when I first used ESET for my PC, it had only 256 MB. And it worked fine!
I managed to upgrade it to 1GB few months ago.
Since then, I tested: 1 US product (its founder now is kind of playboy), 2 German AVs, 1 Romanian AV, 1 Russian AV, 1 Czech AVs. (I had a lot of free time back there)
My experience about performance is: ESET is the best, 1 German AV comes 2nd, Czech AV comes 3rd. The other is...