What graphics card can i put on amd athlon ii x4 620

just needing some input, what is the most suitable graphics card for this system?
it is a quad core and has 3gb of ddr2 ram.
games such as arkham city run but sort of in slow motion, so I guess that its in need of a more powerful graphics card.
It has been suggest that I purchase a gtx 650TI.

It is currently running a ati radeon HD 4350 graphics.

Thanks for any advice