ATI doesn't exist anymore (not the original) you kno that Strangestranger, all the head CEOs and development teams were cut. Now its just purely what AMD chose:P
You have to look at the games you like, the 4870 X2 is 30 % stronger than the 280 GTX (games like Grid, which are in favor of AMD/ATI, will prob break that trend), and is the better buy for only 100$ more.
Although it will also depend on ur mobo, if you single card mobo, the X2 with out a question. IF you have sli, the I would weigh my odds, see what games I like:)
For me, I bought the 4870 X2 when it came out because up until the 8800s I was a pretty hardcore fan of ATI starting with TNT 2. all the way to 1950 XTX, plus its performance as a single slot card is pretty fierce.
Look at some benchmarks, and see the games you like and what games that are coming out that were programed on the hardware.
Then make your decision.
Example Crysis runs better on 280 GTX than 4870 X2, same with GRAW 2, there are a couple more including far cry 2 (for now). 4870 X2, grid COD4, and alot more SLI/Crossfire loving games:)
Honestly its more of a gaming direction than just a raw power question, if the 4870 X2 was only 1 GPU with the same power, then it would be different answer.
Anyways look @ prices 2, the cheapest can also be a good solution:)