Problem with iMac 27" 2010

Hello,
I have an iMac in 2010, a 27 ° i5 "with an ATI 4400. A month ago I wasediting photos with Adobe Photoshop CS5 and suddenly the computer freezes andnot responding to anything. I was turning a photo and froze image and keys orthe mouse replied. After off and on seems that everything was alright, butafter some exercise retry CS5 complex was re-hang and I can never finish theoperations I want.
The computer only has one year and left me lying in a moment more than ever Ineed for my work. In the Apple Reseller tell me it's the graphics card andasked me by arrangement € 432 if Apple does not take the guarantee. I am aniPhone user, iPhone, Macbook Pro and other articles and I am very disgutadowith this problem. I can do?