2 Video cards or 1?

[Rbadeau]

Ok, so I am not sure what to do here. I want to upgrade my video card and this is what I'm thinking. I love the CPU I have and have never seen it go over 50% unless Rendering with Blender (That does not bother me).

I have:
OS: Windows 7 Home Premium (x64) Service Pack 1 (build 7601)
CPU: 3.40 gigahertz Intel Core i5-3570
Mobo:ASRock Z77 Extreme6
Hard Drives: 120 GB SSD(OS) and 2 x 500 GB HD Western Digital (Games and storage)
Ram: 16 GB Dual Channel
Video card: Radeon HD 7850
PSU: XFX Pro550W

I play Wow, MWO, Teso, and a handful of other games not as polished.

I want to go with either a single Geforce GTX 970..(won't have to upgrade PSU either)- $350ish

or

get two Geforce GTX 960...(and have to upgrade my PSU)- $500ish

or

a Geforce GTX 980..(I'd rather not but if it is better than two GTX 960s and can do it without changing my PSU, I probably would go that route)- $550ish

Cost is not really an issue give or take $100-$200.

Ultimately, I don't want to see any chop or any type of video deviation. More or less just because. Lag happens but I'd like to see flawless gameplay if possible.

What would be the better choice? Any advice could be helpful. Thank you

[David (PassMark)]

I always prefer 1 slightly faster card, rather than two average cards.
One card will nearly always use less power, make less noise and be more compatible with various games.