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Usually a company releases a high end and a slightly lower end fairly close together. A 30X/90 would be a direct competitor to the gtx 970 and should be £200-£300. I just hope they don't make the 290X the 970 competitor.
Are you asking if a 680 will tide you over untill the 3xx series? I think it would.

What are 680 sli like.?

What are 680 sli like.?
I had sli 680s a while back and they were pretty strong.
anything has gotta be better than buying a gtx 970Many say wait till x300 series . Going on recent times my guess at best they will go head to head with say a 980/970 and use more power and heat and noise will become a issue. I do hope I'm wrong but if AMD's cpu's are anything to go by we may well be disappointed.

If you can get a good deal on the 680 go for it, otherwise just pick up a 670.
If you have no brand preference my money would go on a 7970.
Assuming your buying second hand.