There seem to be (I haven't investigated this very deeply) differences in the style of xorg.conf, XFree86, etc based upon what method you use to configure them--that is, doing X -configure produces a different sort of file than does running xorgconfig.
In my case, I usually just run xorgconfig. (I should add that I know my monitor's and vid card's specs.) For a step by step, bsdnexus.com has an article in their wiki.
I'm giving the main page's url because the configuring X page has parentheses in its url and isn't rendering properly. However, go to the bottom of the main page and you'll see a link to the configuring X article.
This of course, doesn't actually answer your question, but if you follow the steps there, you should be alright with your mode lines.