1 6970 is a lot faster than 2 5670s. 2 5670s would probably perform somewhere between a 6770 and 6850 at absolute most. With cards that weak, I would also imagine microstuttering would be a huge issues with 2 5670s. If you need a lot more graphics performance, definitely save up for the a faster single card. Depending on your resolution, a 6970 might not even be needed. A 6870 or 6950 will perform quite well on all but the highest resolution displays. What resolution are you playing at, and which games are you playing? Also what sort of power supply do you have? You may have to upgrade your PSU to run a higher end card.