I see. Then 6870 will do. But, if you don't have a certain budget and are willing to go a bit more expensive, then a 6950 would be great. It will give a performance boost and stay at about the same power consumption. You can also check what the nvidia camp has to offer, but that comes down to personal preference.
So yeah, 6870 for ~$200 and same performance as 9800gx2
6950 for ~$260 and about 20% better performance.
Personally, I'd go with 6950, just for a bit of future-proof.