How much RAM for Minecraft server?

Well, if it's like you say, just a simple survival server it shouldn't take more than 1-2 gb at the most. The RAM for a server depends on how many plugins and players, so if you little to no plugins, and a small amount of players, you shouldn't be taking up a whole lot of memory.

The thing that sucks about the PCs servers is if you'd want the server to be lag free, you could see if Gabe's idea would work because I agree with him about the GB thing but make sure you do not get your hopes up on the server, because sometimes you might have to have a certain amount of RAM, so if you can't find one easily, and it's challenging to find the right parts for your PC including the screwing and unscrewing the parts. Also, some parts could be stubborn too. So, if you'd want it completely trust worthy on the lag free server, you'd have to save up a lot of money for a PC that already has good parts like 16GB or more. I experienced changing the video cards with one of my PCs and let me tell you, it is a pain in the neck to find the right parts for your PC. I've finally found the right video card and the thing needed to bend some in order to fit.

It shouldn't need much. 4 GB would be plenty