The 3870 X2 is fundementally two 3870 stuck together. So two of them would be like have four HD 3870s. They do however, have slower memory, but faster clocks.
Their drivers at the moment...aren't the best. But that's the ATI way- the cards will get more and more powerful as they age =]
I was thinking about getting two...but I'm most likely getting two 9800 GX2s.