If you want mercy you might need to start doing the research before posting - searching for 'JDK' in google would have netted you part of the answer, at least. In any case, JDK is for developers, but it includes the JRE. Unless you're going to program in Java you can stick to the JRE. Java 7 works fine for both client and server for me, this is the output of 'java -version':
java version "1.7.0_01"
Java SE Runtime Environment (build 1.7.0_01-b08)
Java HotSpot 64-Bit Server VM (build 21.1-b02, mixed mode)
You were asked how much free ram you have, by the way. Then follow this guideline: