Is there a difference in SEO when naming pages with "_" vs. "-" For example stock_bulletins.aspx compared to stock-bulletins.aspx?
Yes. "-" is considered a space while and underscore is not. Go with the dash. It's a rather minor issue though, so I wouldn't go making big changes if you've already got something up with the underscore.
there have been several discussions of this in several other threads. Do a search for them and you'll learn more than you want to know about the difference between hyphens and underscores. But yes, the preffered method is hyphens.
As what has been mentioned by the other members, you may also want to refer to our DP Google FAQ for more details.