How does multiple GPUs installed work?

[Comdrpopnfresh]

If one were to add an older, yet still capable, graphics card to a single-gpu system (all ati/amd gfx cards for simplification), how would that work? I have no interest in CF/SLI/HYDRA'ing cross-generational cards, but will any functionality be limited in either card for independent use?
For instance:
I have an old but high-end model 4870 laying around and am contemplating adding it to a system with a 6950 to boost my mining horsepower (and maybe provide a workaround for mining and viewing video without seizing up the entire system and having to hard reset), OR putting it into a different desktop with a 5450 in hopes of gaining some gfx muscle for HD/high-res video viewing on a large display/monitor/tv while solely using the 5450 and maintaining low-power for normal computing usage.

Can multiple gpus be used independently with the same function of each as if they were alone? And can the secondary graphics card go idle/off so that it isn't eating watts and producing heat sitting there while the primary alone is used? I don't want to add heat or power consumption to the system any time the computer is running, just when I am actually using the dang thing.

 

[cusideabelincoln]

The secondary card should go idle, but it won't shut off. The 4870 has a high idle so it's not a good choice if you don't want to add extra heat and power consumption to a system.

If you add it to your first system you can use the card for mining. I don't know how well it performs but you can use its capabilities.

If you add it to the second system you would have to hook the monitor up to it for you to use the extra power it has over the 5450 to watch videos. But your idle power consumption will probably go up at least 50W; for the things you want to do with that system a single card makes sense and a secondary card will add no benefits. You're probably better off just upgrading the video card to something like a 6450 if you find the 5450 is not doing the job you expect it to do.