I plan to install windows on the SSD and everything else on the Caviar Green HDD. I plan to SLI two GTX560 Twin Frozr II cards together. The NZXT Zero case, an Antec 650w power supply, a CD/DVD drive and a 250 gb hard drive are the only old components I intend to keep from my old rig.
I would go with the Z68. Besides that everything will be compatible and work fine. Just research installation to hard drive default rather than SSD. I tried and nothing would work so now I have Windows on HDD and Linux on SSD. Also have you considered GeForce 580s?
If you're not gonna video edit or do 3d modeling you should go with the i5 2500k. Amazing processor that surely won't bottleneck those 2 gpus. 12gb of RAM won't give you any moremperformance than 8. I suggest you get G.Skill Ripjaws 8GB DDR3 1600MHz.