yes you will get noticeable performance increase , and HD 5750 is defiantly will run 100% on that PSU u mentioned
the different between HD 5750 and 5670 is 25W
. Max Power Draw for HD 5750 is 85W
and its depends on the card but tier are much nvidia card than ati eats alot of power
as for the upgrade , you should mention your monitor as well what res you playing with .
FPS 35+ more depends on the game