Im sure most people on here will tell you to go for a single card over two if you are starting from scratch. I havent seen benchmarks but i am sure performance will be close. Keep in mind the top end 670s will have faster speeds than a reference 680, but this will be dictated by your price range. Benefits of the single card is that it uses less power, creates less noise and heat, and some programs can have issues supporting SLI. Also keep in mind if you get the one better card now, you can add another one of them down the track.
As long as you understand what you are getting into going SLI you'll be fine. I'm running a GTX 580, which I upgraded from SLI 460s, which was an upgrade from SLI 260 216s. Never had any real issues with either SLI setup. Micro Stutter can be an issue, but if you don't plan on multi screen gaming, my guess is you will keep your FPS high enough. Everyone complaining about SLI profiles hasn't run SLI.
I literally just bought a GTX 660 to replace a dying 460 and plan on grabbing another as soon as they drop under $200 a card. Edit: the SLI is to replace the GTX 580 not the 460. Just wanted to clear that up after reading the post.
If you are at all nervous about running SLI or the potential issues you will run into grab a 670. If you are comfortable with SLI and want the most performance go 660 SLI. Either way I have a feeling you'll be happy.
By the way I've went the "best card you can get and add a card later" route now and it's a fallacy. Unless you have a lot of expendable loot, by the time your top end card drops to a reasonable price there will be something much better out there. For example the GTX 480 is just hitting $200 and is completely worthless compared to $200 cards now. Adding a second 480 would not be a reasonable decision when looking at performance/cost/power. Unless for some reason you have a 1000w PSU you bought that was overkill in the first place...
Went ahead with a GTX 670, and will probably buy another one when price drops around Christmas sales, that should last me great till the generation after the next; will use a GTX 260 right for right now as a physx card too. Thanks for your reply and link to benchmark; was really curious about the performance of those cards SLI.