During the Unigine Heaven benchmarking, the Phoronix Test Suite also monitored the GPU core temperature from the binary driver for the HD 4870, HD 5770, HD 6870, and HD 6950 graphics cards. The Radeon HD 6950 temperature with the DIRT 3 cooler had an average temperature under load of 66°C while starting out from idling the temperature was at a respectable 39°C.
The AMD Radeon HD 6950 is certainly a very powerful graphics card that will more than handle today's Linux desktop needs when using the proprietary Catalyst driver. In many respects, it is overkill considering today's lack of demanding OpenGL applications/games for Linux, but it is certainly future proof and coming up will be more demanding native OpenGL games for Linux. At the same time, the actual price of the Radeon HD 6950 graphics cards aren't too steep with retail models shipping for $240+ USD.
In terms of open-source support for the Radeon HD 6950, it is there, but you will need to build your own driver stack for the next couple of months until the various components work their way into released Linux distributions. Even when that happens, you will likely be disappointed in the performance with the open-source Gallium3D driver and the power management abilities that do not match the Catalyst driver or in terms of overall features. If you are buying a high-end graphics card, the proprietary drivers are -- for better or worse -- your best option for the near future.
Regarding the Sapphire Radeon HD 6950 DIRT 3 graphics card, its current retail price is $270 USD. This DIRT 3 version is at a $30 premium over Sapphire's standard Radeon HD 6950 model with the cost difference just getting you a copy of the DIRT 3 video game and a larger heatsink. Unless you're a Windows user (or using Wine under Linux, the DIRT 3 edition isn't of much value with the game and the stock heatsink cooler for the HD 6950 should work just fine since there isn't much value to pushing the graphics card harder under Linux. Regardless, Sapphire Technology remains a trusted AMD AIB partner and one of the brands we recommend at Phoronix due to their very good reputation, competitive pricing, and being one of the vendors kinds towards Linux through their support of Phoronix hardware samples to deliver hardware and Linux driver tests.