I use Bitdefender on a second generation i3 with 4gb of RAM (my father in law's computer he doesn't want to upgrade
) with no problems. The only way to know is testing it.
The impact of any security software on system performance really depends on a lot of things, from whatever else you have running to driver versions installed. That's why we get the never-ending discussions of product x slows down computers/no it doesn't in forums. It's quite relative to what you are running.
Of course, it's much easier to test it on a few personal computers than a few hundred computers in a corporative environment, so, make use of those trial periods