You must be crazy to buy the GX2.
1. More money
2. More heat
3. More noise
4. More Power to feed
4870 is about 13% slower from 280GTX by the last Toms review and 280GTX by the same review is slightly faster than the GX2 (we are talking average). My guesses are 4870 is going to be about 10% slower and the gaps close even more if not using 2560x1600 4xAA due to the 512 MB compared to 1GB of the GTX.
Also 4870 is single GPU compared to the GX2. It just doesnt make any sense to buy so much trouble for just 10% (which can get even lower after driver updates and new games which could not be optimized for dual gpu)
Also there are games that are still not tuned for dual gpus.
Price wise, Price / performance wise 4870 is WAY much better.
From what I learned with all my experience with computers during the last 15 years, don't underestimate the problems you can have so try to minimize it and buy smart not by heart