Help with deciding on wich RAM to add / can I match different CAS

[Olphu]

I'm having a hard time deciding which memory sticks to buy.
I already have these:
Corsair XMS3 6GB (3x2GB) DDR3 PC3-12800C9 1600MHz http://www.overclockers.co.uk/showproduct.php?prodid=MY-205-CS&groupid=701&catid=8&subcat=1389

I need to add (if possible) or replace them with on of these kits.
My options are these:
Corsair Vengeance 12GB (3x4GB) DDR3 PC3-12800C9 1600MHz
http://www.overclockers.co.uk/showproduct.php?prodid=MY-294-CS&groupid=701&catid=8&subcat=1389

Corsair Dominator 6GB (3x2GB) DDR3 PC3-12800C7 1600MHz
http://www.overclockers.co.uk/showproduct.php?prodid=MY-270-CS

Corsair Dominator 6GB (3x2GB) DDR3 PC3-12800C8 1600MHz
http://www.overclockers.co.uk/showproduct.php?prodid=MY-269-CS

(Any other suggestion)

Can I add any of the above kits to my existing Corsair XMS3 6GB (3x2GB) kit, even though the CAS might mismatch??
With the first kit I would have 18GB of ram, with the other two kits a total of 12GB of ram.

I use my system for video, photo and audio editing so I need all the memory that I can afford.
On a regular job I have open Lightroom 3, Photoshop CS5, Premiere CS5 and After Effects CS5 all at the same time.




My system:
Win7 Ultimate 64bit
i-7 930
Corsair XMS3 6GB (3x2GB)
Gigabyte X58A-UD3R