Dan01 - 9:48 pm on Nov 24, 2012 (gmt 0)
Google now considers underscore as a word divider so it is an option.
The way I remember it - Google sees the underscore as a space.
Never use a space though. I remember (this happened several times) when a link is transferred (perhaps via email - I can't remember right now) that the space was replaced with several characters (like %@&). When that happened the URL was unusable and gave a PAGE NOT FOUND. I was with a friend who had an email with this. I replaced the characters with a space and it worked. But how many people will do that.
I guess that bottom line is: the underscore is the best.
As per the extensions: I don't think it matters to the search engines. I don't think it matters as much as the dash/underscore debate. BUT... I still use the .htm for the reasons I said above.
Before I used a CMS (and I still might use simple HTML pages from time to time) I would store the pages in a folder with images. Sometimes the images would be the same or a similar name. An htm or html would help me quickly identify the webpages from the images and other files.
I don't think there is a reason to use .htm or html for SEO.