Any 980 Ti should work. However, it is always best to get the same card for SLI since sometimes the different GPU speeds can cause issues. The drivers are supposed to default to the lowest speed on both, but that sometimes doesn't work. That can be gotten around by manually setting the same GPU clocks on both cards via PrecisionX or Afterburner.
 
EDIT: With your 4790K, both cards will run at X8 speed as long as you have nothing else taking up any of your CPU PCIE lanes. If you have any other cards (wifi, NVME SSD, etc) taking up CPU PCIE lanes, SLI will NOT WORK since less than X8 lanes will be available to each GPU - Nvidia require X8 lanes minimum per GPU for SLI to be enabled via the Nvidia Control Panel.

Edit2: Unlinking the two cards to adjust the GPU speed independently to match them up may be required to get them at the same GPU speeds.

The chipset has PCIE lanes as well -- Arestavo just noted it's the PCIE 3.0 CPU lanes that you don't want to take up.
http://www.intel.com/content/www/us/en/chipsets/performance-chipsets/z97-chipset-diagram.html
If you look at your manual for the mobo, it should help tell you how to set it up.
 

hah, indeed we did, and i even added an edit that was redundant:) glad it's figured out.

I don't have SLI'd 980 Ti's (just 1 card), but I've read on this forum about mis-matched 980 Ti GPU clock speeds causing instability and setting them to the same speed by unlinking them and manually setting them at the same speed resolves those instabilities.

arestavo
I don't have SLI'd 980 Ti's (just 1 card), but I've read on this forum about mis-matched 980 Ti GPU clock speeds causing instability and setting them to the same speed by unlinking them and manually setting them at the same speed resolves those instabilities.

mine are off set speeds and no issues what so ever, i have overclocked them as well, even stock they are fine though, one card runs 1450 and one card will drop randomly to 1448 here and there lol