Get the 670 and just light that $100 bill you saved on fire...or take the less literal route and burn it all on the 680.

Here's the current Valley 1.0 leaderboard- dual gpu category (updated today)

Notice how their are tons of 670's smoking 680's sli'ed

I went from 670 FTWs to 680 Classifieds, and I came away with an even stronger opinion of the 670 FTWs (I was really high on them before so didn't think that was possible). Granted, I had some great 670 FTW's (all 3 would game rock steady at 1306 MHz), but I had to overvolt and OC the snot out of the 680 Classy's to see any real meaningful improvement (sure, you can see a 4 FPS increase on a benchmark but can in no way feel that in real life).
 
You are getting a very minimal improvement for $100 more. By very minimal, I mean that the difference between the 670 FTW and 680 is so little that in absolutely no situation will you find yourself that the 680 can play something the 670 FTW can't. Frankly, if you get a good 670 FTW it will be faster than a lot of 680's. My 3-way 670 FTW setup scored a 3DMark11 score high enough to place it 4th out of what would be 10 3-way 680 setups on the 3DMark11 leaderboard thread here on the forums.
 
You can certainly buy 680's if you want and know that in absolute terms you will likely have a faster card than you would have had you bought 670 FTW's, but how much faster? Probably not much at all.

Had a small typo, I meant my 3-way 670 FTW setup would have placed 4th of 10 on the 3-way 680 chart. Accidentally typed 3-way 680 which I never had.

That is correct because every card has a different personality and some will clock higher than others
 

SeanEJohan

My first 670 FTW oc's to 1333mhz in 3Dmark11, my second card is more of a average 670 FTW, oc'ing to 1280mhz. 

670 sli 1346/3650
http://www.3dmark.com/3dm11/6413819

670 FTW 2GB no question. that's what i went with and loving it. haven't touched the clocks.  out of the box crunches SETI/Einstein@home with no errors @ 1175/3105, 1188/3105 with a bump to the fan. gaming is a pleasure with it, and it's nice and quiet. if i didn't care so much about anti-aliasing i would have gone with GTX 660 Ti FTW.
 
i saw 4GB can actually hurt performance slightly. unless your using three monitors at uber high rez 2GB is fine.

So jealous of people that get great 670 cards.  Mine only go to 1202/3629 and 1189/3629.  Fantastic cards either way though.

The GTX 670 FTW's are a match for the 680's, only they cost less. They also have better coolers than the regular 680 cards. I have a GTX 670 FTW Signature 2, easily stays below 70c with a custom fan profile, even when overclocked. Best card available for the price right now imo. Buying a second one soon.

Yeah the 700 series are supposedly coming out soon. According to what I've read the GTX 770 is about 5-10% faster than a current reference GTX 680, that places it at around the same amount faster than a GTX 670 FTW.
 
The Step-Up program is good but you're forced to buy the cards from EVGA directly and I can get them significantly cheaper elsewhere (£70-80 difference on the GTX 670 FTW's). The 5-10% boost is nothing a quick overclock on a 670 FTW can't resolve, so I'll be sticking with my dual 670 FTW's until the 9 series at the earliest. I don't see much point getting 7 series GPU's (which are still running on a revision of Kepler) if you've already got decent 6 series GPU's (670 or higher) installed. If you're only running a single card I can see the benefit but if you're running 2 it's already overkill =) Only Crysis 3 and Metro Last Light will challenge an SLI'd 670 setup. Other games may challenge CPU wise but not the GPU's.
 
Once they are released I may get a GTX 990, but until then I don't forsee any game for the next 3-5 years that won't run nicely on dual 670 FTW's. Crysis and Metro excluded of course, though they're designed to be difficult to run on purpose. Your average game will run fine on ultra settings with any single GTX 670/680, let alone 2. I'd rather turn something like AA off in 3 or 4 years than pay out £650 for a couple of new GPU's =)