[Build Help] getting a graphics card

so i decided that building a whole new computer was unnecessary since i already have an old HP (thats been sitting in a closet for close to 2 years) and tried to see how well it ran. well it has 6gb of memory which i was quite surprised to see, an intel pentium E5300 @ 2.60GHz and this motherboard. the power supply was pretty weak (230W). now it didnt have a graphics card and i saw that the mobo had the PCI x16 so i bought a 400W power supply and a GEFORCE GT 620 graphics card, all from best buy for around 120 bucks. probably the worst waste of money ever. ran BL2 worse than my laptop and struggled to play Planet side on all low and about 1200x800 resolution...

now i need a graphics card that doesnt need more than 400W and can play Planet Side 2 BL2 on medium graphics at around 1600x1200 resolution atleast. i have minimal knowledge on graphics cards so i need some help. is it worth buying a new graphics card or should i just scrap the whole thing and buy a whole new computer?

thanks!