http://www.3dmark.com/3dm/5874997? this is what i got on the 2nd try.
What 980 did you get? Which model i mean? and how much did you pay?
You say you're selling your 680s? If you decide to keep the 980 i mean? How much are you thinking you'll get out of the 680s? If you can get $300-350ish or more out of them then it's definitely worth it to upgrade to the 980. As people mentioned, in artificial benchmarks a single 980 is about equal to two 680s but in actual games you get about a 5-8% increase in overall performance with the 980. And there's also more advantages with the 980, you have much lower power consumption, it's quieter, has double the vram etc.. i've even read a review on newegg for the Gigabyte G1 gaming gtx980 where he says that it gets about 15% better fps than his GTX690 which is the same or a little better than two 680s.
But honestly if you're just wanting the best performance you can get for gaming then i would cancel the 980 order and just get two GTX970s. The GTX970 is about halfway between the GTX780 and GTX780 TI in overall power for gaming, it typically gets about 3-5fps more than the 780 and usually falls about 3-4fps behind the 780 TI in most games (although it actually beats the 780 ti in a few specific games). The real high quality GTX970s such as the EVGA FTW+ or the Gigabyte G1 when overclocked (they can easily hit 1,500mhz most of the time) will actually TIE with the 780 TI for pretty much all 1080p games, at 1440p it does drop 1-2fps behind, and at 4k the 780 TI definitely gains a decent 5-6fps lead though. But overall you're looking at almost the power of a 780 TI with a 970, and even though it's technically only got 3.5gb of full speed ram you may be better off going with two 970s rather than a single 980 or 780 TI.
You also get the same maxwell technologies with the 970 that the 980 has, such as MFAA anti-aliasing which gives you full 4 x MSAA quality but only takes the fps drop of 2 x MSAA. So even if the 970 is 3fps behind the 780 TI in a game with both using 4 x MSAA, the 970 can kick on MFAA and gain an average of about 10fps instantly. Yes two 970s is more expensive than a single 980 ($550 vs $650-700ish) but overall gaming performance wise you'll be getting 10-12fps more than a 980 will give. Now the one exception to this is if you plan to do heavy 1440p gaming, like 120hz or doing very vram instensive games at 1440p like shadow of mordor with the HD textures, dying light, star citizen etc.. , or especially if you plan to do 4K gaming. In that case you will definitely be better off keeping the single 980 since it has a full 4gb of full speed vram vs 3.5gb of slightly slower vram with a random 500gb chunk of super slow GDDR1 speed ram on the 970. Or if you don't mind going with AMD you could try the R9 290X, they're on sale quite cheap; some as low as $300. Look up Lutro0 on overclock.net marketplace if you're interested in AMD, he's selling FORTY reference R9 290 cards for $200 each, and if you buy 2 or 3 he discounts you down to $175, $150 etc.. per card. Nvidia is definitely better cards overall but for the price AMD is a good deal if you plan to get 2 or 3 cards. You could also check out the R9 295x2, comes pre-watercooled and quite powerful for only ~$50-75 more than a 980 and has two R9 290Xs on the single card.