So I'm getting ready to build a new system and I read the latest review on the 4870 X2.
I was really just trying to decide between 1 4870 X2 or 2 4870s in CF, but as I was reading over the results I couldn't help but notice how well the 9800 GX2 was performing. I play at 1680 x 1050 with AA (Results here)
Comparing the results from those charts with these prices (from newegg)
To warn you, 9800X2 has higher maximum and average frame rates than 4870 (single) but it's minimum FPS can dip below 20 and that can mess up your gameplay. I wouldn't advise on a 9800X2 SLI setup because quad SLI is not that great compared with the other options, it's scaling is far from perfect.
I suggest going towards single 4870 or 4870X2 for your resolution for smooth play.