I think there's definately a war coming, because Voldy isn't nearly as "terrible and greater" than before...
I suppose it would most likely happen at Hogwarts (although, someone suggested that it would happen at graduation ceremonies, which I find unlikely, seeing as we've never seen graduation ceremonies, even when Ron's brother "graduated")...
I think there's a reason Grawp will stay around Hogwarts. I think there's a reason the giant spiders are still there... In the beginning, the giant squid seems like a scary sea monster kinda thing, but we have found that it likes the students (at least some of them... Fred, George and Lee were even tickling it). Wonder if it could turn ugly on unwanted invaders.
I think there's a reason the Thestrals are there, and that we've learned about portkeys and floo powder. I don't think Dumbledore or any other staff would want 11-year-olds in the way of danger, so I think the majority of the students would be sent into hiding, if the school fears an attack is immenant (sp?).
So, younger or less experienced students are evacuated. I don't know if
all students will be evacuated, and the members of the DA (and I think all of them) would stay against the wishes of the staff, or if the staff allows some students who feel as though they want to or should stay to do so.
Voldemort's army would include, I assume, Voldy (naturally), the Death Eaters, most likely several of the youngsters who were formerly students at Hogwarts (I don't think Draco and Lucius would be on opposite sides), the Dementors (man that's gonna be an ugly fight), probably the giants.
What about the goblins? Have you noticed how many goblin revolts Harry and Co. have had to study about in History of Magic? Have you noticed how very little of what Harry and Co. have to learn about is not used later? If they decide to go to the good side, they could be incredibly helpful, but if they decide to join Voldy (which I don't really feel like they would do) they would be incredibly dangerous enemies.
I think the Order of the Phoenix would definately be there, possibly with the exception of a few people who are helping to protect the younger children (I see Molly more in this kind of role, rather than fighting against Voldemort and the Death Eaters herself... it's a very important role, too). I do think this will be the end of Dumbledore, at the hands of Voldemort (their powers rivaled at the height of Voldy's powers last time... if he's more powerful than before, then he would be an even more dangerous match for Dumbledore). It's likely Lupin will die, most people seem to believe it'll be at the hands of Wormtail, I personally have no idea. I don't think it likely that all the Weasleys will survive.
In fact, assuming Percy is short for Percival (I feel almost certain it is), there are some interesting things to consider could happen...
Percival was indirectly responsible for the deaths of an uncle and his own mother. I draw the analogy that Sirius is the "uncle", so is Molly's time short?
Anyway, sorry I went off on a rant. Yes, definately a war coming, likely at Hogwarts.