New Graphic card(s)

I don't think you want crossfire.
Your motherboard will only run the second slot at X4. That is bad for crossfire, slowing down both cards.

A single strong card will be better anyway.
1) It will avoid any microstuttering issues
2) It will not need such a strong psu to run just one card.
3) You will have less case cooling issues.

So that means I should go for a single 6970 instead or can you recommend something better for the same price?

A 6970 goes for $330 or so.; not such a great deal.
An equivalent performing 7850 will be about $250 which is one heck of a better deal.

The GTX570 is also a comparable performer in the $250 range.

Keep your eye out for the GTX660 which should be a good card. specs are unknown ad so are prices. But Nvidia wants to compete better in the $250-$300 range.

Is my motherboard Nvidia compatible?

Yes, at least for single graphics cards, they need a x16 pcie graphics slot and sufficient power. It matters not if the card is amd or Nvidia.
Since you are using amd now, a change to nvidia simply means installing the nvidia graphics drivers.

One last question (thanks a lot for the replies btw, they are an amazing help for me):

On the Toms Hardwares review of the HD 7850, it says that it is unable to run some games. is that going to be the same problem for me?

No guarantees, but I doubt it.
The review was done very early in the product cycle.
Since then, there have been several driver updates which have fixed all sorts of issues.
That is to be expected with any new product.

If you are worried about a particular game, google with the game name, 7850 and "problem"