Earlier last year when I was having problems with the video cards that were put into my machine against my wishes, I spent a fair amount of time talking to XFX about their video cards and the effects that heat had on them, their (the techs) experiences with overclocking them, and a hoist of scenarios.
The biggest key that I was told was airflow, airflow, and more airflow. If your case can not get airflow into that card, then if it does hit higher tempo ranges under load, it will not be able to cool down. This leads to the obvious, thermal shutdown.
My computer setup had a further issue as it had 2 6970's in crossfire set up. The set up of the machine, while very clean, simply did not allow for enough air to enter into the case to cool everything inside. There was a self contained liquid cooling set up for the CPU, and between that and the rest of the normal set up, they simply could not stay cool after continued load.
This is where I finally got into a "frank" discussion with one of the techs, and while he said he could not recommend it, he mentioned that his cards in his gaming case were all liquid cooled, and the performance was night and day. Since that discussion, I went a bit over the top and really liquid cooled the hell out of my machine. Now, it has the correct XFX cards, they are liquid cooled, and it runs very well. I have never had a performance issue from heat since the day I set it all up and got it running properly.
Keep in mind, that liquid cooling can be expensive, and usually voids your warranty. Do not buy "gently used" parts, it is a waste of money and time as they will have some issue, sooner or later. Lastly, get a good monitoring system to set up with it and monitor temperatures, I use a Koolance CTR CD-12 that montitors everything and will shut down the system if a problem arises (like a gently used pump failing or a "like new" radiator springs a leak) it audibly warns you and shuts the system down.
