I served in the Peace Corps in Central Asia for two years, and I will go to law school next year. Several of my fellow volunteers are already in law school, and those I know of were accepted by some very desirable schools (Georgetown, Harvard, etc.). Depending on what you hope to specialize in, and the feelings of the admission committees at your chosen schools, Peace Corps service may be a plus on your law school applications. If you complete your service and conduct some good, solid projects in-country, it can show that you are tenacious and have good problem-solving skills. If nothing else, it is likely to clarify your ideas about "helping others."
That said, service isn't for everyone. It can be difficult, lonely, and frustrating at times. Some of my friends quit early, thinking that their time would be better spent at grad school or working for money at home. For me, the positives outweighed the negatives. If it is something you have been interested in for a while, now is the time to do it. You may find that you'll get more out of law school after a couple of years' experience, anyway.
BTW, one of my best friends served in Paraguay; for the most part, volunteers in South America *really* enjoy the experience.