Yes, the votes are purely symbolic, but the political impact is vital. It will force members of Congress to take a stand on the record and thus establish how much or how little support the war still has. Assuming the Democratic whips know how to add, I rather expect that the Dems can get a majority in both houses or they wouldn't be calling for the vote. There's a fair amount of debate as to how much Congress can do to stop the war, short of cutting off funds completely. However, this symbolic vote is a vital first step for future action.
We'll see what President Bush has to say tonight. His staff rotations make a lot of sense, particularly sending General Petraeus to Ira (although he'd be better used in Afghanistan at this point). The General is considered by everyone I've heard from to be the U.S.'s top counter-insurgency expert. He literally wrote our book on the matter. Can he win it? I highly doubt it. Maybe if he was put in before or shortly after the civil war started in earnest. Alternately, if the President and Cheney resigned and handed him the job he might well have a chance. I do hope that he can make the best out of a bad situation, even if victory isn't an achievable outcome.
I will also note that I don't have a beef with President Bush rejecting advice of his top generals in launching the 20,000 troop surge. There's no way that many troops and the new reconstruction aid will make a difference, but that's a problem with the merits of the proposal. Bush's old line, that he just did what the Generals' told him, was an abdication of responsibility. It was never really true as such, he was more than willing to fire those that disagreed with him, but it was an actively dangerous line of argument. Troop levels, small vs. big footprint, is ultimately a strategic and not just an implementation issue. Generals can and should provide advice on such strategic issues, but ultimately the civilian leadership has to make a decision. Why am I harping on this point? Well when we rip into the President for a surge that can't work, we shouldn't buy into his original argument that he is somehow taking orders from his subordinants. Instead we should demolish it on the merits while noting that no-one credible, including Bush's top generals, thinks that a 20,000 troop surge will really make a difference.