I think 7870 is a better option here. If you wanna go for Crossfire then you need to consider medium range GPUs like HD 6850, HD 6870, GTX 560 etc. Crossfire of value segment GPUs like HD 7770 or HD 6770 are not recommended because
i. Their performance is not enough to handle games @ highest settings.
ii. The amount of money you'll save for not going for a high end single card will be lesser than the amount you need to spend to get a CF/SLI enabled motherboard and a premium PSU.
iii. You'll also need a good and big cabinet for it.
If you go with cards like HD 6870 or GTX 560 Multi-GPU then it is okay because at 1080P resolution a HD 6870 still performs better than HD 7950.