As someone who likes SLI and Water Cooling, the 690 is looking like a bit of a bargain lately! Consider, two 670's at £320 each plus a pair of water blocks at £100 each and you're looking at a £840 spend. Turn those 670's into 680's and you could be the wrong side of £1,000!
Now, if you shop around, you can pick up an ASUS 690 for £700, add to that a SINGLE block at £100 and you have an £800 cost, and arguably a neater and easier build.
Ok, two discrete 680's would generally beat a single 690, moreso when you start overclocking. However, how much over 60fps do you really need? lol.
So, I say a 690 today, if you can get one for the £700 mark - an ASUS one was UNDER £700 a few weeks ago at a certain etailer, it went up to a staggering £710 pounds recenty! lol - can be a bit of a bargain, especially if you plan to water cool.
If you're staying on Air it's less competitve, but can still make your life a lot easier when it comes to the build. Also remember that a 690 has a PCI-E 3.0 bridge built-in, so if you're on an older PCI-E 2.0 board there are potential (slight) benefits running a 690 vs. say a pair of 670's. Also note the less power consumption vs. two cards, making a PSU upgrade less likely.
For me, if I had to buy now, I'd get a 690 and a waterblock rather than two individual cards. Thankfully, it being post the expensive, erm... festive season, I'm happy with my pair of GTX 570's for the moment.
Oh, to those who say "overkill" you're not wrong, however my old Q6600 @ 2.4 with 8800GT plays EVERY GAME I WANT just fine, provided I turn some settings down. I don't wanna turn stuff down! Lol
Scoob.