If you bought the 7970 for $550+, you will lose at least $100 in resale, as you're gonna have a tough time selling it for more than $450.
IMO not worth it... just get a second 7970 for $450 and overclock both of them to 1125/1575.
Though so too. I bought 7970 in the second day of its release for $600. Got sight of you're having 7970 Lightning, I was thinking to get it as a 2nd card. Or should I just get another reference one and OC it manually as you said to max CCC settings? What would be a difference between max overclocked reference 7970 and max overclocked 7970 Lightning with NO voltage touched?