A factory overclock is irrelevant and not something you should pay for. You can overclock your HD5770 as well.
@omgitz a second 5770 is over a hundred quid! this will cost me an extra 60.
When over 100 = double the performance and 60 = 15% increase they aren't even comparable in terms of value. That you aren't talking about $60 but rather 60£ only makes the cost larger and the upgrade even more questionable. Like everyone is saying it just isn't worth it.