For our main Ventrilo website we're using the _ between words. However, now that I'm using WordPress for our main hosting website it seems the - is preferred. I really can't comment if either is preferred over the other by search engines but I have seen discussion on blogs and articles about using the -.
I am looking for some advice on this... If I have an article about data center network design, which page name should I use:
3. something completely different
4. it doesn't matter
I wasn't sure if there is a difference between using a _ or a - but you guys would know better than I.
I would choose #2. Search engines these days recognize hyphens as word separators. They also seem to work better with hyphens then underscores.
Something you should keep in mind also is that order has weight. The shorter, precise, and closer the keywords are to the domain name, the more weight your keywords have. Avoid having long URLs that are mixed with letters and numbers. Always rename your URL to what describes best the page.
For the #2 it looks fine, it is precise and describes exactly what your webpage will be containing. This is why I chose #2 as best for you.
Hope I answered your question, if you have any other question, feel free to ask!
I also suggest option #2. Hyphens and underscores are treated separately by Search engines. An underscore makes the word a complete string, whereas a hyphen acts as a separator. So it makes sense and has more SEO value to go with option #2.