You don't have any bottleneck with two 670 gtx with an i5-3570k, the psu if it's a good one(e.g. seasonic) it can do for two cards without shutting down/burn/throttling with 650w.
If not, you have to buy a good one with ~650-750w.
You should be fine, but, just curious, why do you need it?
My single 670 can max out almost any game at 120fps. Spending $400 to turn on supersampling in the Witcher 2, to see no benefit at 1080p, seems kind of silly.
(And if you're playing on three monitors, I'd advise going with the 4GB vram versions of the card, just for safety.)
^ honestly i'm inteeested about this. afaik 32 bit program will only use 2gb of ram max. to use more the program must be made for 64 bit. or does the 32bit program have some kind of 'awareness' to let the program to use more than 2gb of ram if more ram available in the system?