[DeltaMac]

I will feed you the "party" line. As Apple support would suggest.
The problem is not with the Nvidia 8600M GT, per se.
There was a block of failing chips, which Apple replaced as a courtesy to their customers. Other computer makers apparently have also struggled with this same chip set. The repair did replace your logic board with an updated, supposedly fixed version of the that 8600M GT. It's a newer build-version of that chip, and supposedly has fixed the issues that have appeared on those models.
Have you seen anywhere that a 'repaired' MacBook Pro failed again?
If yours was repaired with the 'fixed' version of the 8600M GT (to a v2 or v3, I believe), are you having problems again?