1st 690 SLI(quad gpus) is actually not recommended by many. Too much horsepower, little vram. Its like having the power of the Sun and lighting a 100w bulb. Not the best reference but you get the idea..
Coming from a good 690@1176/1800 down to a 780@1254/1752 in gaming the difference is a lot more than reviews and benchmarks shows. As I said elsewhere, I love my 780 but purely for gaming the 690 is a lot more powerful.
No doubt the 690 culprit is the vram and the throttling but FC3@4K ultra settings/no AA with my 780 uses 1750mb vram(avg 45-50fps), I can say the 690 has still a lot of life in it. So assuming someone doesn't go over-board with AA its 690>780.
No back to the OP's situation. Imho I would just find another dirt cheap 670, oced if possible both cards to 1250mhz and call it a day. Best bang for buck and I'm inclinded to say performance wise too.
BUT, if you can sell you current 670 and add the price you would had paid for that 2nd 670 and with that total you can find a used 690 for the same or just a bit more($50-$80), then go for it just for the better hardware sli that the 690 provides.
Otherwise SLI 670 beats 780 anyday and also might beat/match a 690 since most don't OC well. So my vote is go for 670 SLI and wait until Maxwell and see what those bring in the table.
P.S: Don't forget AMD's r9-290/x if you don't mind waiting or going Red. They might surprise us and I would definitely recommend them over a 780 and even a 690 if the reviews favor them well. Also 4GB vram seems really sweet too!