Run the 280's. Seriously, they are much slightly better than the 4870's. Also, unless you intend on a really nice monitor, or plan on playing on HUGE resolutions ALL THE TIME, you would more likely be better off with a single card. Realistically, more cards means higher resolution, and occasionally worse performance than a single card. I'm sure many people will disagree with that, but just look at the benchmarks guys.
Also, if you are running the i7, you would be better off using intels new mobo. And as one last recommendation, get 1600 mhz ram, otherwise you won't notice much difference from DDR2 ram.
Please consider these points. Also, this computer is DAMN close to the new one I will be getting.