Oh boy...
Short Term:
1. It is probably about 5-10% risk if you follow the directions and understand what you are doing.
2. Performance difference between 6950 and 6970 is ~4.5% in FPS and 3.5% on degrees (F). That is not worth it to me.
3. If I bought it from a store that would take immediate returns, such as BestBuy and planned on using the card for 1-2 years I would do it. I would not do this from an internet purchase or savvy store.
4. I'd say that this is an intermediate tech task. It isn't something you should do unless you have prior experience in overclocking or modding. On the other hand, it isn't like building a home made peltier cooler and overclocking by 1.5GHz.
Long Term:
Consider that the practice of downclocking weaker components has been common practice since the Celeron 300A in 1998.
Think of it this way. I don't know the threshold for a core to be considered in prime condition, but lets pretend that it must be at 90%-100% capability.
All 6950 and 6970 cards have two cores. The first core is always going to be prime condition for both cards. In the 6970, the second core will also be in prime condition. In the 6950 though, the core may be significantly diminished and may only be at 70-89% capability. It functions perfectly at the lower stock clock speeds, but over time, it is being severely strained if you push it beyond those limits. Once again the % I use are not by any means the real ones, but they give you an idea as to how the cores are distributed and what the outcome could be. Only ATI and manufacturers know how different the strength of the cores actually are.
Since I use flat panel monitors that are 60hz, I always put on vertical sync. That limits my output to 60fps. Both cards can push 60fps on most games, so it isn't worth it in my book to push the lower card.
Articles I read about it:
http://www.techpowerup.com/articles/overclocking/vidcard/159
http://www.reddit.com/r/gaming/comments/erzl4/buy_an_amd_radeon_hd_6950_make_it_a_hd_6970/
In my personal opinion, I would rather have an nVidia card right now. I currently run an ATI Radeon HD 5850 and it is great, but I like nVidia's drivers better and while this is subjective, I feel like I see stupid glitches with the ATI card that I never saw with my prior nVidia one.
