ASUS EAH6870 1GB Radeon HD6870 Thermal Image Analysis
 

The PCB behind the Radeon HD6870's GPU did get to over 72C, and it washed over a large area of the card with temperatures exceeding 60C. These numbers themselves aren't of much concern, but it does imply that the actual GPU is even higher. An aftermarket cooler may not be necessary, but it should lower those readings by a fair amount.

http://www.bigbruin.com/content/hd6870thermals_1