Multiple monitors don't really dent a decent gpu much at all, especially in a Windows environment at 1080p. A gtx 750ti could easy run 3. That said, you'll need to decide just where your 'light gaming' goes. I can 'light game' easily on my 660ti,but it's stronger than a 750ti and I don't play Witcher 3 or gta:V on that pc.
Then comes price. With the new gpus out and prices of a 1060 @$280+, and a 1060 will top a 980 in more than a few games, the resell price of a 980 has plummeted. You can get a gtx950 for @ $150 new, which is a decent buy if on a handicapped budget, or a Rx 480 for $230 which is about equitable to a 980 in performance.
So, by the time you sell that 980, pay for a decent replacement that fits your needs, you stand to make @$50-$100 unless you find the exact right buyer who'll pay more for an sli setup than buy a single powerful gpu.
For the loss in performance, ability, expandability etc, you'll really end up loosing value by downgrading the gpu.