1) Looking at some information on that PSU, its label does not list information on maximum combined wattage. Also, it lists 230V input, not a range. These suggest to me it's a cheap PSU; I wouldn't trust it for much more than half to two thirds of its label.
2) +12V rails are not strictly additive. The total current available in a case like this might be something like 40A, with each rail current-limited to 28A. You can pull up to 28A from either rail, as long as the total amount is no more than (in this case) 40A. This PSU doesn't list the max combined, so see #1.
Even if overrated, this PSU should still have enough juice for a HD4890, although I certainly wouldn't try running a pair of them on it.