well a 470 is faster than a 460, but a 460 can overclock pretty high and almost reach a 470 stock in performance. however if you are really gonna overclock and overclocked and unlocked 470 can go beyond the performance of a oc 460
bacially either card will do almost all games on the market today at max setting at up to a 1920x1080 res... but if going for a higher res like 2560x1600 then i'd go 470
at stock settings on 1920 x 1080 a 460 should run most everything out at max settings.
some very heavy games like metro might need medium-high settings but only a handful of titles, if thats your goal the 460 woudl be fine now and for a year or so. the 470 would probabyl give ya 2 years of future proofing at maxsettings at 1920x1080 res for said period of time