Graphics cards arent my forte Mark.
As far as I am aware you can run an 8x card in a 4 x slot(I know this for certain as a friend is doing just that).
What I cant see on either of those graphics cards is the voltage that they run at and I do know that this has to be the same as the AGP slot from what previous people have said that they have had problems with.
Can you chek with the board supplier which card will work with it,or find which card has the same voltage(1.5v)as the AGP slot.
That board says it has :
One AGP (Accelerated Graphics Port) 1.5V 4x slot