Depends, If you have a 670, SLI capable motherboard and PSU and can get a second 670 for ~£100 then it's going to be better value than selling your 670 (~£100) and buying a single 970 (~£280), so ~£80 more for a 970, which has less power but more memory. I run 680 sli @ 5840x1080p and do run out of video memory on games like Far Cry, you just have to turn some of the setting down to keep it running smoothly.