Crysis 2 has yet to be released so we can not tell you for certain what its performance will be like on any setup. But if I had to guess I would say yes.
A 700w of a good brand should cover crossfired HD5870s fine. Whether crossfired HD5870s is a good idea depends on the system you plan on putting them in.
If you would like to save some money I would recommend OCing a pair of HD5850s instead for $200 less. The performance difference will not be very large(5-10%.)