Is my card broke? Please help!

Hi i bought a Gigabyte Radeon HD7770 last Monday and it's been crashing my games games like gmod sometimes arma 2 . Some times it just shuts down and says entering power saving mode screen goes black then like 4 mins later it's comes back and i also get reallt bad lag spikes in dayz from 40 fps to 1 even when i put down the screen re and make the settings on low. I have updated my drivers to 8.982 witch OC Guru 2 says it's the newest and it still happens i think it may be that i only have 2gs of ddr2 ram
specs not the best but it does the job not a hardcore gamer
Video card : Gigabyte Radeon HD7770
RAM : Unkown brand of ddr2 2gs
OS : Windows 7
CPU : Intel(R) Core(TM)2 Duo CPU E7400 @ 2.80GHz
PSU : Antec Neo Eco 450C 450W Power Supply
Have i got a dud card and should i just rma it?