Am I right in thinking that the cards stack up as such:
TOP END
AMD 5970 / NVIDIA GTX 580
HIGH END
AMD 5870 / NVIDIA GTX 480
UPPER-MID
AMD 6870 / NVIDIA GTX 470
And (correct me if I'm wrong), the AMD cards generally run quieter and cooler and draw less power? The 480 is a few % faster than the 5870, but bang for buck, the 5870 is a better card?
At what point would moving down in the order of these cards have a drastic effect on my gaming frame rates and future-proofing?
I'm struggling with what to do here big-time!
Yup pretty much what you have stated there
Personally for me, I would not touch the 470 or 480 due to the noise, power and heat and would rather have any of the ATI cards over them.
If you where considering nvidias cards, the ones, which I would look at, would be the 460 and the 580.
Personally for me I don't buy the best there is (generally the second best or what ever suits me best and the best performance/price), once it starts to struggle with games daily, not just one or 2 then that's when I would upgrade, so IMO all those cards that you have mentioned there should last for about another 2 years and the 580, 480, 5970 about 3 years and a bit more, considering all the PC games now a days are poor crap ports. But generally most people on this forum upgrade there GPU (even if it is top end) for every new release

, even if they can still play games at max settings above 50FPS
Just looking here
Dreadnought vs
Viper. Stick in a 40GB SSD, Windows Home Premium in both, and a 6870 in the Viper. The difference is £180 and the Dreadnought contains the 480.
Obviously the 480 system offers the better performance, but is it the better value? The only difference other than the GPU is that the Dreadnought has a slightly more powerfull power unit.
In all honesty if I where you, I would save the money by getting an i5 and 4GB dual channel (1156) setup instead of the i7 and 6GB triple channel memory 1366 setup and put that towards a better monitor, or just save it and put it towards something else. As you really will not see any improvement for gaming between the i5 and i7 and the RAM type etc. Plus also intel are bringing new chipsets out again soon next year so even the top end 1366 chipset will be outdated and no matter what rig you get it will last you for a good 3 years and by then, there will probably be something much better out in terms of RAM or CPUs.
You would notice much better performance in games by saving the money by getting the i5 and 4 GB and putting it towards the top end card, whether that be the 580, 5970 or if you get a cheapish mid-high end card like the 6870 and sell and then buy the 6970 as in real world performance for gaming there is virtually no difference at all between dual and triple channel, 4GB and 6GB, i7 and i5 CPU, you would notice a much bigger improvement by getting a better GPU as obviously that is more what games use really.
Further to that - what sort of FPS differential would exist between the systems (at a rough guess).
In all honesty there isn't really any difference at all between those 2 excluding the GPUs as they are actually pretty much the same hardware, just slightly different motherboards and PSU etc.
I haven't looked at any 6870 expert reviews in detail, but apparently it is suppose to be nearly as quick as the 5870, so I would say that it should be able to handle all games at max settings till you decide on what you are going to do when ATI release there new cards.
BTW, just to give you an idea, my setup is:
i5 750
4GB G.skill RAM
HIS 4850 512MB Turbo iceQ4
1280x1024
All at stock clocks.
Now my setup blasts through all games on max settings (although for some I need to keep the AA and AAA down to around 4x and 8x) and the only game that I struggled playing on max settings was metro 2033 when it first came out, however I heard that it was updated to improve the performance etc. so it would probably run a lot better now than what it did when it was first released on max settings.
Also all of the COD games, including black ops, I can play on max settings including the max AA and AAA, granted though that I am on a quite a low resolution, but my card is 2 years old now and still going strong
I am looking to upgrade my monitor to the dell 23" IPS hopefully very soon and will wait till ATI release there new cards, in order for the prices to decrease for the 580 (hopefully the 570 will be out as well) and will be looking at either the 6950 or the 570. I play a fair few games, but not as often anymore, main ones being FPS, third person action type etc.
Also generally I wouldn't worry to much about the FPS in games in the sense that no matter what system you get from what has been suggested so far in this thread will be able to run games over 45FPS and personally for me I think it is pointless IMO people saying a card is crap just because it can't run over 70FPS as the chances of you actually noticing the difference is very slim as I certainly don't notice any difference at all when playing games on my setup and when on my friends rig, which consists of:
i7 920
6GB RAM
295 2GB
1680x1050
And neither does he when playing games on my rig compared to his, if you ask me I think he is kind of ****ed off that he spend all that money on his rig (his one cost like £1700-1900, although he also got a 1TB hard drive, case, motherboard and PSU as well) when compared to my rig

, which cost a lot less.
The only time you would notice any stuttering or anything would be if your FPS dropped from 50+ to like 25-.