Also i highly suggest you pick your 670's wisely not all are on the same level i would be all over that Asus 670 TOP for its high factory overclock, improved cooler,heavily modified PCB, with an efficient VRM design to produce low temperatures, improved voltage regulation circuitry, much better ChiL controller , specially binned chip and voltage control and monitoring support that you won't find on most other non-reference cards. It looks aggressive to me and indeed is quite a monster let's keep it on the real not only does it violently murder most of AMD cards period it does it without even breaking a sweat most cards bow down or wish they had legs so they could run in fear when 670 TOP enters the room all bets are off.
In that case it wouldn't be a good idea imo also Asus Top is a dual slot card it would be compatible depending on your mobo just personally i wouldn't do it the evga ftw is not on it's level it's a great card don't get me wrong i assume the EVGA 670 FTW is not to far behind the TOP but also it would look funky with mixed matched cards imo lol.
Yep get another EVGA 670 FTW like i mentioned it would be possible but Asus Top is a triple slot card it would be compatible depending on your mobo just personally wouldn't mix and match the two in a sli setup i think it would also look horrible lol just my thoughts it's up to you but my vote goes to getting another EVGA 670 FTW
If you don't have the CPU yet, it would be better to get i5-2500k/3570k instead, you could save some money and those beasts perform better than locked i7 CPU. If you have i7-2600, however, then it's fine . It will be more than enough for SLI GTX 670s.