Hi mate!

To the pont;

- Sell 1 x 6970 (21187-00-40R) and Buy 2 x Identical 6970 (11187-00-40R) ----> Then I can use same water blocks!
Would work.

Sell 1 x 6970 (21187-00-40R) and buy 2 x GTX 580 -----> ¿?¿?
This would be very expensive. 2 WB's + 2 GTX 580 3GB's.. Hmm

Keep 1 x 6970 (21187-00-40R) and buy 1 x 6990 -----> ¿?¿? What I don't like about this setup is the use of different blocks again...
- Also very expensive, but coops with your current card.

This is my two cents;

I don't think you will get much for a 10 month old card, really. maybe 50 - 60% percent of new price. I think you will be better on waiting for GTX600/HD700 that is due in Q4 or early Q1 this year!