Dash or Underscore Better?

Is there any good reference that points to either a dash (-) or an underscore (_) being better than the other for search engine likeability?

Example:

http://www.freshdames.com/paris-hilton.html
or
http://www.freshdames.com/celebrity_news.html

The same question for images. Should I try to break up image names for better ranking, likeness, etc?

Should I go with
http://www.britney.freshdames.com/spears/pregnant/pages/spears_pregnant_jpg.htm

or

http://www.freshdames.com/katie_holmes/wallpapers/pages/holmes-bw_jpg.htm

Does it even matter, for that matter

 

Search the forums, it's been asked a billion times.

 

The summary for those with ADD:

 

I read about a underscore & hyphen test they did for Google if your interested have a look here

 

I agree, a dash is better than an underscore

 

For those who gave actual answers, thank you very much; the reference was a nice add too!