since the gtx 970 3.5 gb card goes for $300, I guess I would be comparing performance with the gtx 970, but I can't find benchmarks of the 4gb gtx 680 anywhere. 2gb sli gtx 680's seem to loose when gaming @ 3x1920 surround, but I think their Vram would be the limiting factor here. I would be playing @ 1080p or 3x 1080p, no 4k yet for me.
specs:
i7 960 4.1ghz
16gb 1600m ram
radeon 5870
875 watt dell psu
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amazon link to specific 680's: (I'm getting 2 for $300)
http://www.amazon.com/EVGA-GeForce-DisplayPort-Graphics-04G-P4-3687-KR/dp/B0083YF55U/ref=sr_1_1?ie=UTF8&qid=1431022223&sr=8-1&keywords=gtx+680+4gb
specs:
i7 960 4.1ghz
16gb 1600m ram
radeon 5870
875 watt dell psu
edit
-----
amazon link to specific 680's: (I'm getting 2 for $300)
http://www.amazon.com/EVGA-GeForce-DisplayPort-Graphics-04G-P4-3687-KR/dp/B0083YF55U/ref=sr_1_1?ie=UTF8&qid=1431022223&sr=8-1&keywords=gtx+680+4gb