2 GB is just split in half for each PCB so you only have 1 gig of memory.
Don't forget that you can get 2 9800 GX2s for the price of 1 4870 X2.
But since your starting from scratch I'd go with the 4870 X2. I mean theres no point going to old tech if your going aim for top of the line.
Oh yeah and the 4870 X2 is about 30% on average better than the 280 GTX, so don't listen to ppl that say marinally, its only in some cases where it matches the 280 GTX SLI. Most of the time it lags behind, but the whole point is that you save like 300$ going with 1 4870 X2 and to be able to sometimes match or even comes close to 2 280 GTXs is insane for 560$