In the strictest sense, this WOULD be plagiarism, because you wouldn't be acknowledging your true source for the Hippocrates quote (the Princeton Review book). Now, if you were to actually go and take out a book of Hippocrates' writings from the library and read it, you could use the quote with a clear conscience--but in that case, why not find a different quote and use that instead? It would be more interesting as well as avoiding the issue of plagiarism.
Besides, quoting Hippocrates in your PS is a pretty big cliche. (The same holds true for other well-known authors, philosophers, etc.) I own a book called "101 Tips on Getting into Medical School" by Jennifer C. Welch, who is the director of admissions at SUNY Upstate. Tip #39 says, "Do not quote Robert Frost in your personal statement. Be original. Quotes from Robert Frost are very popular on medical school applications. You should probably leave him out of the process [...] just be creative, original, and yourself." Instead of Frost, you could substitute Hippocrates, Albert Schweitzer or anyone else equally famous. The bottom line is that adcoms are interested in reading YOUR words, not those of a well-known author.