Make post titles thicker/bolder?

My site is http://www.red94.net

I am using Vigilance Pro.

I would like the font on my post titles to be thicker/bolder, similar to the font used on the post titles in this blog ===> http://andrewsullivan.theatlantic.com/

(that blog is also using the same font as i am.)

How do I do this? Note: I don’t want to make the text any BIGGER; only thicker/bolder.

thanks.