Conventional wisdom is if Democrats take one or both house they'll start a series of investigations on Bush Administration.
While some will Bush for that, I'm betting the Democrats will start throwing lots of money, for better or worse, on the War instead.
Because they'll fear a successful terror attack between Nov 2006 and the 2008 elections, and the last thing they'll want is to be attacking Bush; while America attacked.
Instead, they'll push for more weapon systems. They'll follow a policy of Muscular Isolationism with some token multilateral diplomacy. Token because the partners know the can't count on the neo-isolationists in Congress.