Many 7900GTX cards (not GTO or the shorter GT versions) are equipped with "defective" Samsung memory, IIRC, that are prone to failure within a few months of usage.
Not sure of the percentage of those cards that failed, but it could be as high as 20% from what I have read on the forums. A bit like Crucial Ballistix 2 GB DDR memory kit (including the Tracer series) that has virtually 100% failure rate within 1 year!