Hmmm... the ATI has said that 3800 and 4800 are middle range cards, so do not expect high end results, but 4800 should definitely be better than 3800 so, it should be nice card, although it can not be compared to g200 (if it will be released any time soon... and if 4800 is good, then we will see G200 soon enough.)
The saying that: "the 4870 is supposed to be about as fast as the 3870x2" is from ATI so it should be somewhat right. It's much more than I by my self am expecting it to be, because the differences between 3800 and 4800 series are not so big. Most likely 4870 can get near the results of 3870x2, because it's one chip solution, so there is not the scaling problem.
In titles where scaling is good, the 3870x2 will be better, and where the scaling is not so good, the 4870 will be better. In most cases where the scaling is even somewhat good the 3870x2 should be much faster.
It will be near 8800GTS I supose, then the price most propably is the thing that means a lot in that competition. First released parts are with GDDR3 because of low suply of GDDR5... So at the beginning, it's very close fight.