Current items that will go in this new rig:
2 x 6950
8gig 1600mhz ram
and misc harddrives and what not.
1. Sell the 2 x 6950 cards, not worth the increase (if any) and extra heat they produce. Plus Crossfire headaches will loom even larger on a Sandy Bridge system, get a single decent card setup, Nvidia preferably, but your choice
2. RAM for Sandies tend to run @ 1.5v and recommended no more, it's a whole new ball game with this memory thing and SB, but its dirt cheap ATM luckily, so picking up a dual or quad channel set will be peanuts if yours is the 1.65v+ variety?.
3. I'd go for the 2600K (like mine lol) over the 2500K, as you have the hyperthreading which handles an overclock a lot better than my old i7 920 ever did, and the money you make from selling the rot will more than pay for the difference between the 2500 & 2600.
4. As for the board, well, it depends, I got the Asus Z68 v Pro Gen3, but barring the extra SATA ports and nothing else worth mentioning, you would be good to get the Asus Z68V at much less the cost if you dont need these useless features? All in all, a great board, and fantastic at clocking
If you make good money from selling stuff, then grabbing an SSD (SATA3) is recommended if you dont have one.