I think the lowest it will run on is 2Gb (I think 4Gb is the lowest officially supported) max being I think
128Gb.
How much resources it uses while running depends highly on how much is available. Kernel Task will probably use anywhere from 400Mb to 1.5Gb depending on how much ram you have installed. All the other system stuff will also vary in amount of ram used but usually under 2GB. I think after a fresh boot on 8Gb I'm usually around something like 4Gb used but I also have a bunch of apps that launch on start-up. Once you start doing stuff Mavericks generally keeps your mem usage pretty high by leaving app data in a cache, I'd say anywhere from 65-99% is pretty typical at least for me.