What you are investing in is a dual GPU card, which means two GPUs on one PCB, or SLI. This of course, carries both the advantages and disadvantages of running multi-gpu's and even though you will get more raw performance out of the DUAL 460's, you will have to worry about balancing out heat and power consumption.
It is usually adviced that, when given the option, one should choose a single GPU setup even though it may be a bit lesser in performance, to eliminate the possibility of common multi-gpu issues like micro-stuttering, scaling, a need for a better PSU or CPU bottlenecking.
As far as driver optimization, the 460 is somewhat of an old card right now so I would guess that it scales pretty well across game and probably has sufficient driver support.
If you're in dire need of GPU performance, the 460 will give you the most power. But for future-proofing, I'd say go 570, your getting better architecture, better temps, less application optimzation problems and good driver support.