7970 is a high end card, therefore it's most likely to have full bandwidth of 16x/16x to work %100 without bottleneck. If you want a board that has x8/x8 (PCI-E 2.0 mostly) I'd recommend you to get x16/x16 instead.
Yeah but if as deadjon said the difference is only a minimal 10% I'd stick with the 8x speeds. For now anyway I'm going to use a single card which will run at 16x and that should be more than enough for the time being. If I want a dual 16x I would also need to get a x79 board which runs on the overpriced i7-3960 since the intel socket 1155 boards I'm looking at don't apparently support dual 16x speeds.
Btw the slots will be 3.0 dunno if that makes a large difference to a 8x speed 2.0 slot...