At this point, I would suggest either a gtx480 or a 6950. The slight performance premium on both the 6970 and gtx570 do not justify the extra cost. If you are going for FsX, I would suggest a gtx480. for everything else, I would suggest a 6950 unlocked.
The gtx560 was instrumental in bringing the prices of AMD's cards down, but that has left it in somewhat murky waters itself. I wouldn't buy one as of this moment, unless the price is lowered another £15-20, as it is not as fast as a 6950 at stock or overclocked and only has 1gb of ram in it
*edit* on the subject of unlocking a 6950, it is safer to unlock the shaders than to flash the bios to a 6970, as the 6970's ram runs with different timings, as well as running a higher vcore. The issues that Tute found I'm guessing were people flashing the card.
I agree with you when you say that the 570 and 6970 aren't really worth the extra. They don't have much of a performance increase for the £. For a heavy o'c though a 570 would be a star im guessing. After reading around, only unlocking the shaders would give me a 1-2% increase. Given the lack of risk due to the double bios switch, I'd most likely take the chance with the flash. Besides, overclockers are now advertising the rev 1 asus 6950 as 'flashable to a 6970'. I'll just make sure I have good airflow in my case and don't overclock any further. Then i'd put another in xfire

Does the 2gb of ram on the radeons not help in fsx no? Kind of thought that it would