Well, for now you can look at these two web pages I found:

ASUS Radeon HD 6870 CrossFire
ASUS HD6870 Crossfire EXCLUSIVE Review
HD 6870 vs HD7850 Comparison

At a glance, I would say that the CF will be more powerful than the HD7850, except in some games where the CF doesn't work properly.
I think the performance of the 7850 is similar to the 6950, we're talking about 1GB. So in the end, both with the CF and the 7850 you'll have a limitation of 1GB of memory…

I would opt for a HD7870 minimum, but if you can stretch for the 7950. If you're not getting much for the 6870, it's not a bad investment.

Now, the same thing also makes it profitable to get a 6870 second-hand and for not too much money you have almost double the power of what you have.
I wouldn't pay more than 80-90€ for a 6870 second-hand and always from someone you can trust.
I mean, I don't see the point of buying a new one for 140-160€ when the 7850s are at that price…

Anyway, remember that you need a PC that can take good advantage of that CF and have a decent power supply

Best regards!!

The crossfire of 6870 is clearly superior to a 7850 but you have to take into account that your power supply (and motherboard of course) support it, in addition to the extra heat that implies having 2 graphics mounted. To make a crossfire it is not necessary that they are of the same brand, even they can be different models as long as they belong to the same generation (6XXX, 7XXX).
To compete with the crossfire of 6870 you should already go towards a 7950/7970, depending partly on the optimization that the game offers for crossfire.