Need to upgrade graphics card lp

Hi, trying to learn the basics of computers so please bear with me,
My son has an acer aspire x3450' w7.
Amd athlon 11 x4 645 quad core pro
Amd rad hd 4250 graphics
6gb ddr3 memory
2tb hd
Has a pfc 220wts
Problem is wants bf3 i've spent some time searching different forums to learn he needs a low profile graphics card and have found sapphire hd radeon 6770 lp would play medium results without having to change the wts (185 max) please can you give me some advice if this is so, before i buy thanks really appreciate the help