Keep the gx2 and wait 6 weeks till Nvidia releases the card to take on HD4870X2, its going to be alot more than just a die shrink
A very trusty source, who told ous the performance of 200 series, has said the 55nm GT200b will arrive shortly after the HD4870X2 and it is much more powerful than what everyone here is giving it credit for.
Most here are convinced the 4870X2 is the end all, and Nvidia wont be able to compete.
My advice to you is wait 2 weeks after the 4870x2 launches then sell your 9800gx2.
2x gt200 55nm with 25% higher clocks, with the chances of some additional units ( TFUs/TMUs ) is what I have heard.
Ati know this.