The X1600 configuration on the late 2006 MacBooks and iMacs is dodgy and a replacement, which will only have a limited warranty, will likely fail in time as well, unless you religiously keep the computer running a lot cooler than intended. Otherwise the continuous exposure to the fairly intense heat will eventually damage the new processors. Thus this has to be done before the damage to the hardware is done. Once you have issues it's too late. Better spend your money on a new machine. Apple are unlikely to help you with this issue. They haven't helped any of the other hundreds (if not thousands) of people affected by the faulty X1600 GPUs. They know very well that the configuration on these MacBooks and iMacs is bad but it would cost them too much to fix it. Cheaper to ignore the matter, even if you're likely to lose a few customers over it. I did a Google using the words X1600 and petition. Maybe you should do the same.