I'll use it to toy around with the highest OC with the watercooling!
I'd get my hands on Far Cry 4 and Assassin's Creed Unity, get completely lost in those amazing graphics!
If I win, I will replace my Radeon HD 6850 and play more video games, do graphic design and watch videos with my kid.

I am building a new watercooled media center/ steam box.

Current parts include
i5 4670k
Asus Maximus VI Impact
-bitspower waterblock to be ordered
240GB Sandisk Extreme
Corsaid Vengeance Pro 2x4GB DDR3 1600
Gigabyte GTX750ti
Silverstone Raven RVZ01
Silverstone ST45SF-G 450W

Currently, there are no plans to make a waterblock for the card I have, so long term plan would be to replace it so it can be added to the loop.

If i win, I will chose the Poseidon and hook it up in this machine.
I will be using it to replace my aging computer so that it can play the most up to date games when I am not able to play in surround!
I'd sell it.......HoHoHo.

I don't care which one you want to give me.:D
Build a rig for data visualization, and maybe some mining for fun...
I would happily put it in my main rig of course :)
I would then donate my my current card to someone less fortunate.
I would give it to my 14 year old nephew who has spent the last year and a half saving up money to build a nice machine so he can pursue his career choice as a game programmer. :)
Finally build a new PC, since I'm still running a ASUS Crosshair (yes, the first one), with a GTX 260.
Gaming like hell. Let me have that delightful card and I'll put use it to stream my steam and make it scream!
Replace this 2 year old 7950 and game 24/7 with it!