I'm not considering my 580 a waste...because I am still going to be playing modern games maxed out....its just that IF gog throws out a big promo soon, I might just end up going old school...but of course I don't want to just waste electricity like that...so thats why I asked
Okay so it looks like my best bet is to disable SLI in nvidia CP and leave everything as is
Do you have numbers on how much electricity you actually save by taking out the one card? Because it seems to me that the one card isn't going to make a huge difference in the total power consumption of your building. But that just seems logical to me, and so could be totally wrong.
If the card is slotted but not used, it will draw the small amount of power needed for it to sit there doing nothing but being ready to respond to commands. The idle power for the GTX 580 is not specified, but has been measured at approximately 30 watts.
So taking it out will save you about 30 watts, or about 1 kilowatt-hour for every 33 hours operation. It will actually be somewhat more than that, because your power supply is not 100% efficient.
The wear and tear on the card from being repeatedly slotted and removed is considerably greater than the cost of electricity for leaving it in. Unless the OP leaves his computer powered up 24/7 and lives in a place with atrociously high electric rates (say, Denmark), his decision to leave it in place and idle is the correct one.