Road to Perdition, LXG, A History of Violence and Stardust were published by DC imprints, but the creators retained film rights and sold them to studios other than Warners (DreamWorks, Fox, Lionsgate and Paramount respectively). I don't think the box office returns of those films will have any bearing on decisions made by Warner Bros about greenlighting DC films, especially DC superhero films which don't have much in common with the aforementioned four projects (other than perhaps LXG, but that was simply butchered in the translation).
As for WB's success with DC projects itself lately, Catwoman flopped and will hopefully serve as a reminder to them of what not to do, while most of WB's DC films thereafter have done well. Grossing 2.4X production budget is generally a very successful return that paves the way for a quick sequel (if it's a franchise film) and will yield the studio a lot of profit. Batman Begins and V for Vendetta crossed that threshold. Constantine came close to it, grossing 2.3X its budget, meaning that it yielded a good profit, but perhaps not quite enough of one to take the risk of making a sequel (although a sequel is still possible). SR, while grossing a good amount by the standards of the genre, was obviously a disappointment in terms of return on its budget.
Of the four Marvel films released in 2006/2007, X-Men 3 and Spider-Man 3 grossed above the 2.4X threshold, while it looks like FF 2 will gross just below it (at about the same return as Constantine) and Ghost Rider had a return (1.9X to 2.07X, depending on whether the budget was $ 110 mil or $ 120 mil, both of which have been reported) which was just mediocre.
