You cannot crossfire a 5770 and a 5850. You can crossfire cards from the same family (5750 and 5770, or 5830, 5850 and 5870) but you cannot cross family lines. If you want something more powerful down the line from a 5770, just get a second one. 2 5770s in XFire is generally equal to or faster than a 5870.
Edit: SLI must be 2 identical cards. CrossfireX is anything within the family, though identical cards still work better.