I seem to be having problems with my iMac's GPU (ATI Radeon X1600 - 128 MB in a 2007 white iMac 20" - Intel Core2Duo 2.16 MHZ / 2 GB RAM) : color lines intermittently appear when I play video, glitches appear when I move windows around, etc. I was wondering : 1) Is it possible to replace the GPU in such a computer? 2) If it is the case, would replacing such a crucial part of the computer be worth it, or would money be better spent on a brand new computer? 3) If I chose to replace the GPU, would anyone have a suggestion as to which model I should buy as a replacement? Any help would be greatly appreciated!