3870X2 is scheduled for Q1 2008 release ... can you wait that long?
Anand's benches have shown 1.48-1.7X performance when cf worked under oblivion and UT.
So, 3870x2 should have at least the same performance as 2x3870.
Benefits of 1 3870x2 card, is that, you can add another one later.
And, obviously, 2x3870 has no more upgrade path.
Personally, I bought the 2x3870s now, then, I might sell 1 and buy 1 3870x2 when it comes out. 3xcrossfire 3870s
Remember, crossfirex can mix cards.