I am thinking of getting an iMac. I will be doing mostly photo manipulation and 3D. I am not a professional, but expect to put quite a bit of time into my work. Apple rep thought I should buy an iMac 2.93 GHz so I could get an ATI Radeon HD 8450 512 MB - Is this good advice? Is the extra cost justified for what a I want to do? I am not a big-time gamer. Thanks, Mark