Upgrading RAM - Different Types?

[Phyre]

I currently have 6GB of the OCZ Obsidian 9-9-9-24 1.65v. I'm looking to upgrade to 12GB but there's none of the same in OCUK and it seems expensive elsewhere.

"Corsair XMS3 6GB (3x2GB) DDR3 PC3-12800C9 1600MHz Triple Channel (CMX6GX3M3A1600C9)" has the same timings, same voltage etc. Will this be ok to put in the same system?

Thanks

 

[Phyre]

It's not currently overclocked but I was thinking about it. Why does occupying all 6 slots cause problems with overclock??