Given your needs, as we have both said, a hd 6k card would serve you best. I wouldn't consider a 5850 for multimonitor gaming at this point as the 6950 makes it an obsolete option, even non reference models such as the saphire toxic edition you have mentioned. If you were only running 1 display and could get a great deal on 5850, then sure but multimonitor gaming needs all the horse power you can throw at it and a 5850 can struggle as is, let alone going towards the future.

The 6970 vs 6950 battle is simple if you don't want to overclock. Get the 6970. If you want to save some money stick to a 6950.

As far as the 5870, the 6950 is consistently a quicker option and only rarely does it not pull ahead by a fair margin.