Two r9 270Xes would out perform a single GTX 770.
However, you would need a powerful PSU (like 650W or higher) and the prices is about equal.
I highly don't recommend doing CrossfireX, you will come across many issues. Just get a single gtx 770.
Well technically crossfire 270x's will give you better performance but crossfire has many issues. I would recommend going with the most powerful single card that fits your budget that way you can always sli down the road. I have a Asus GTX 770 and it handles anything I throw at it.