The card will not overheat if it was put together properly; the stock cooling system may do a VERY good impression of a hairdryer, and it may not be as effective as an aftermarket heatsink, but it will do the job. As far as the pixels/geometry issues go, that would be artifacting. Generally this is caused by one or more of the following things:
-GPU clocked too high (when overclocking)
-VRAM clocked too high (when overclocking)
-Failing GPU (unlikely, but possible)
-Failing VRAM (more likely than the GPU packing it in, in my experience)
-Overheating GPU
-Overheating VRAM
My gut feeling is that it is not as big of a deal as people are making it out to be. The card comes with a warranty anyhow, so... I'd go for it. And DON'T get the 1900GT. It's MASSIVELY overpriced at $349, and SIGNIFICANTLY slower than the XT.