Glad it works for you Lethaface!
But is a very interesting problem with regard to the other cards.
So me and Lethaface both have the 4870, although Lethaface has the x2 version grr. So it could be this fix only works on the 4870 but this is unlikely I would think its an xp/vista issue.
Ok heres a thought though, I brought my 4870 in december and it seems the 4770 is newer as reviews for it are dated around april 2009. So you could be right about it being newer and causing a problem, even though it has a lower series number. Seems there is roughly a years difference between the cards coming out.