The two cards are in SLI so you still only have 256mb of memory even though each card does have 256mb. Both cards split the workload essentially so yes you do get more performance if the game supports it. You still only have 256mb of memory, because both cards load the same things into their respective ram.
Hope that helps.
And the new 1730 is a really nice laptop. For performance though I think those high end laptops aren't worth it. Id rather have a better performing desktop for cheaper and get a laptop that can do basic gaming if it truly needs to, which in my case is almost never. But to each his own.