I have an i7-920 and an evga 970. I wanted to see what would improve gaming more. Upgrading the cpu to a 960/980/990 depending on price or get another 970 in sli?

Also the 970 I have is the 3978-kr. If I do get another 970 to put in sli, should it be the same card or could it be any of the 970 series cards?

Ditch the X58 and get a modern motherboard and CPU. Upgrading your current platform isn't worth the money. The CPUs you listed are all old and will all hold you back. Getting a 2nd 970 isn't worth the money either. You aren't getting all the performance out of your GTX 970 now; adding a second one is not a good use of your money. A second GTX 970 would choke your motherboard and CPU even more, adds compatibility problems, and is a path towards diminishing returns.

Upgrade the CPU and motherboard and you can gain 20% performance above your current single GTX 970 performance without needing to buy another GTX 970 or change anything about your current GTX 970. Or buy a second GTX 970 and gain 0% to 80% above your current performance. The choice is up to you. If it were me, I would upgrade the motherboard and CPU first so that everything is faster and allow for better SLI performance potential in the future. Why put in a second GTX 970 which will also be choked 20% by the CPU? Why try to max out the graphics capabilities but leave everything else old and slower? And in the future, what will you do? The two used GTX 970s will have lost much of their value by then. You will want to upgrade the graphics cards and will still have an old slow CPU which will be even older and slower by then.

Personally, I'd look into selling the GTX 970 and buying a GTX 980Ti before running SLI. You would gain a lot more GPU power and a little more VRAM. The problem is, your motherboard/processor platform is holding you back either way. Especially if you don't overclock. If you are not very interested in tweaking your system, it's best to upgrade a new platform. To be honest, I would recommend a i3 4370 and Z97 board over what you currently have, it would be a solid upgrade.

Thanks for the replies, all great suggestions. I did forget that the motherboard is a bottleneck at this point. Remember buying it and then 6 months later being disappointed because the new ones with sata and usb 3.0 came out.

Also, I tried over locking my i7-920 but I couldn't do it. I remember it was from the c0 bat hand not the d0 batch that seemed to over click pretty easy.

I play games like Attila total war and even with the 970 it still doesn't crank all the way up to ultra settings. I can move it to ultra settings but frame rates drop below 10. That is why I was thinking it might be the cpu which might have to do to many physics calculations.

I just might need to buy a new motherboard and cpu.