a side note about heat transfer in general... the rate of heat transfer is solely driven by the temperature gradient. By temperature gradient, i mean the temperature difference between the chip and the heatsink, and then the temperature difference between the heatsink and fluid (air). Example: lets say your GPU was at 120*F (your heat sink is (ideally) ~120*F). if the ambient temperature increases by say 20%, your rate of heat transfer DECREASES by 20%. this is illustrated by the basic heat transfer equations:
Q_dot = Rate of Heat Transfer (BTU/s) (Joules/s or Watts)
k = conduction heat transfer coeffiecient (dependent on material, pure materials have higher k, resulting in higher Q_dot, which is why good heatsinks are made of copper)
t = thickness of material the heat is conducting thru
delta(T) = temperature difference (usually celsius or kelvin)
h = convective heat tranfer coefficient (dependent on fluid density and viscosity, geometry, and velocity of fluid)
note the rate of heat transfer is directly proportional to the temperature difference. all things being androgenous (given and constant), the temperature change results in a proportional change in heat transfer. An easy way to counter act this to supply more voltage to your CPU/GPU fan inorder to increase the velocity of fluid across your heatsink, ergo raising your "h" and increasing the "Q_dot" or rate of heat transfer (WHICH IS GOOD!).