The higher thermals on the GTX 260 is because the OCed CPU is outputting more heat than before and thus the ambient is higher in the case as you probably do not have too good of case ventilation.
Actually, my cpu is cooled by a corsair h50, which is a watercooling kit and drives the heat directly out of the case. As for case cooling, I have a NZXT Tempest which has 4 120mm and 2 140mm fans. So, I think it could be because of dust.
By the way: my GTX 260 c216 (factory oc) temps go around 60-66C when playing gta iv or Crysis. During simpler games like Modern Warfare 2 and Bioshock and Batman AA, i get 54-60C. fan speed is always on 60%. are these temps "unsafe"? i don't want to shorten my graphics card's life span. thanks.