As has been written elsewhere in these forums, Marines really need Thunderhawks and/or drop pod assaults to succeed.
My old post [URL="http://www.belloflostsouls.net/2009/02/epic-death-from-above.html"]Death From Above[/URL] covers some of this in the second half. It might help you.
It sounds like you've got 20 stands of Tactical marines--am I right? If so, you've got three Tactical detachments there, to which you can add your Dreadnoughts and drop pod them onto the enemy objectives/blitz. Obviously, this requires getting a Strike Cruiser and, ideally, a drop pod model, but you can playtest this approach without them--the models really aren't all that necessary in gameplay terms.
The Marines "killer unit", though, is the Landing Craft. Load that bad boy up with two Tactical detachments or two Devastators and an Assault detachment plus six Rhino-class vehicles (I like a Pred Annihilator detachment with two vindicators attached), and then drop that bad boy on the enemy blitz to create some major headaches for your opponent. FUN!