This specific point is very important. Design your resume to save time for the recruiter. The recruiter has hundreds of resumes to parse. If you make the case you are a great match for the job opening in the first paragraph your chances of being noticed increase enormously. You are not telling who you are - you are selling a solution to a specific problem.
> If you are applying to a PHP job
There is little point in mentioning you wrote key parts of MVS/370 because the recruiter will have no idea what it is. If the company uses Ruby on Rails a demonstration of your expertise with JBOSS will be (correctly) treated as noise.
I think knowing C makes me a better Python programmer, but unless the job asks for it, I wouldn't mention it before the first technical interview with someone who agrees knowing C makes someone a better programmer.
Tip: using a spreadsheet to keep CV parts saves a ton of time.
Tip 2: find out who the recruiter is. If you can understand more about the person and his/her expectations and background, you may fine tune your CV even more perfectly.