because of the clocks the 5870 x2 is better... the 5970's OC to the clocks of the 5870's...
when you OC they're about the same, except the 5970 is usually cheaper. Single GPU really is the way to go at your rez though though.. I would get something like the MSI 5870 Lighting and push that sucker to 1-1.1ghz on the core... then you'll be rocking and wont have to F*** with CCC or cfx profiles or w/e.
by the time you need the power of a 5970, the next gen will be out :/ this is coming from someone who used to swear by dual GPU, but they really are a PITA.