GTX 970 v.s. 980 v.s. SLI 970

Getting a gaming PC for my son had a few questions





FIrstly this is what I will have



-Intel® Core™ i7-5820K Processor (6-cores, 15MB Cache, Overclocked up to 3.8 GHz w/ Turbo Boost)



-8GB Dual Channel DDR4 at 2133MHz



-2 TB 7200 HDD





On the fence on Windows 7 or 8. More comfortably with 7 but 8 is cheaper....







Here is what I want



NVIDIA® GeForce® GTX 970 with 4GB GDDR5





If I get the NVIDIA® GeForce® GTX 980 with...

GTX 970 v.s. 980 v.s. SLI 970