Got problems after installing AFOX HD6850

I installed my AFOX HD6850 and teh catalyst driver into my system, i rebooted my pc and as soon as my PC get to load up my desktop my GPU's temperature increases as well as teh GPU load and when it reaches 60c and above my monitor turns off and on.. everything was fine with my old GTS450 but now i have this new card i got this problem.. what went wrong?? sorry for my english..   i need answers as soon as possible    

am i under powered??
do i need to buy a TRUE RATED PSU as what my friends said?? i saw it in teh manual that teh card needs at least 350 watts but im using intext accopia 550watts which is generic..

my system:
cpu- intell g620
mobo- ASUS P8H61-M LX3 PLUS
hd - 160gigs sata1
psu - intext accopia 550 watts (generic)
gpu - afox HD6850