Performance between the two setups is quite close, but the X2 consistently manages to beat out the Crossfire solution. However, it looks like the original HD 3870 is going to take a bit of a price drop which is always a good thing.
So if you can get two HD 3870s for less than the X2, is that the route you should take? Yeah, probably; the performance difference between the two setups is minimal and if you're able to get the CF solution cheaper than the X2 one it works out to be a better deal.
The other thing is that if you're not running a PCI Express 2.0 motherboard you can't unlock the full potential of the X2; initial results show some decent gains for the X2 when moving to a PCI-E 2.0 motherboard.
If you're after this kind of performance we would break it down for a few particular groups :-
1) If you're on a tight budget and already have a CF capable motherboard, get the CF setup.
2) If you already have a single HD 3870 then don't bother upgrading to an X2; just nab a second HD 3870 and you're in business.
3) If you're not running a PCI Express 2.0 motherboard but have a CF one on say a P35 based chipset, then it's probably best to stick to the CF solution.
4) Finally, if you need four monitors the CF solution is better when compared to a standard X2.
On the other hand, the perfect user group for the X2 is :-
1) If you have a PCI Express 2.0 motherboard and want Crossfire performance.
2) Money isn't really a problem.
3) You have intentions for CrossfireX.
4) AMD Crossfire on a non-Crossfire motherboard.
Both setups are good and both really do sit in different market segments. Hopefully the X2 is a sign of good things to come from the AMD camp. The next few months will be interesting, not only from AMD but also NVIDIA. The good news is that Hi-Def gaming is a little more attainable now thanks to cards like this which also allow for future support with Quad-Core video card performance.