1) Yes.. it is possible on the new ASUS Z68 board.
2) No... it doesn't really matter 32MB or 64MB cache on hard drives. I would pickup the Samsung Spinpoint F3 1TB.. IMO
3) Your setup as a SSD as primary drive and RAID1 of hard drivers for storage is a good setup. I don't really see anything majorly wrong with the setup.
4) Photoshop... Your PC looks good for use with Photoshop, specially with 16GB of RAM. It will be beneficial for your uses. I agree running the GTX 550 Ti with your system. There is some benefits with quick sync should help with Photoshop.