Best keep to the HYPHENS only, and most definately. SEO sees one word otherwise. Google parses out each hyphen as if it were one space/spacebar typing. With Underscores google treats the word as an extra character, so if google sees kick_the_ball, it will read kicktheball, and this word has no rating in the keyword game. There has been so much debate on this issue and many say it does not matter, but google people say this is the way to do it. Also remember that google is not consistent due to different SEO techniques from different search engines. The same word can deliver three very different results. But if you stick with hyphens you are ok. Better to get a thesaurus though and try to condense every file name to one word, then you are 100% spot on. Hope this helps. Chris.
Google recommends using hyphens, but 2 years ago a Google Webmaster Trends Analyst (whatever that means) said
it doesn't really matter (halfway down the page) and that your time is better spent generating original, compelling content rather than worrying about SEO minutiae.
If I were to start my website again I'd probably stick with underscores for the readability over hyphens.
Oh, and SEO forums are filled with vague, unsubstantiated, unfalsifiable and/or unreproducible claims - even more so than LL MFDB discussions

I found by far the most useful real-world guide to be google's own, linked above. And it's free.