Go with RADEON 5770 HD.
It will perform great at your desired resolution
Will a 5770 HD help boost the graphical performance, mostly hes looking to up the graphics in wow to higher settings than what his 8800GTS allows. Should the 5770HD allow for a decent graphics increase?
Also does the 5770 require a better PSU than a 8800gts, or should it be able to use the same PSU the 8800GTS was using with no problems.