Like I said... If you really want them, get them and don't look back. You wouldn't want to go through buyers remorse like myself (i wish i waited for nvidia 6series haha
While 2x GTX480 may beat a single 680, you have to keep in mind the things mentioned above plus the fact that SLI doesn't scale perfectly. There are games you may get almost double the performance of a single card, and some games which would be less than that. If you go single card you don't have that to worry about.
Also after a quick google search i found this
(provides links to some benchmarks)
They show 480 sli vs 680 is very close in perfomance. Mind you, a 7970 with the new drivers either beats or is on par with a 680, and driver improvements have since improved performance as well since those benchmarks were done. So what's happening is that you're going to pay almost the same as a single new gen card for 2 cards which are only close
in performance to a single 680 or 7970, yet have to pay an extra 100euro (like you said) to upgrade your PSU as well.
You could also get a 7950 for about $300 (250-260 euro maybe?) compared to 670s which go for $340-$380. It will still be a good bump in framerates (overclock it and itll be like you got a 7970), there will be no need to get another PSU, and then a year or so from now you could get a second one for less.