I think the general hope is to have something on the level of Company of Heroes, but with melee combat worked in to be more Warhammerish. Think about how awesome that could look for a second. Think the intro to Dark Crusade going real time. That'll be amazing. At that point, the game itself could be worthless; I would just play to see the war.
Tyranids would be hell on any system if used at that level of detail, but people have been calling for them so much you have to wonder what Relic is going to do. That's especially true when you consider this going to come out in the shadow of Starcraft II. It'll be back enough will all those people simply don't know any better saying it's ripping off the Zerg. It could almost work against them. Thankfully with Dawn of War people are generally wiser to the Warhammer/Starcraft situation, but there will still be lame cries of copy-cat.
My best guess is that I have about one to two years before this hits. I'll have to upgrade then. Guess I better start saving.