...Can I be able to combine my GTX 580 with another GTX card (in the future.)...
...You won't be able to sli with other future gtx card i think..
Well, in 5 years time you definitely wouldn't be able to (or if you could somehow you definitely wouldn't want to). The only way I can see it being possible is if it's like the 8800GT/9800GT (which it wouldn't be since we're talking about a 5 yr gap), which are basically the same card (and then for instance flashing the 9800GT w/ an 8800GT BIOS). But yeah, if you don't plan on buying til Dec/Jan, I wouldn't even worry about it until we know more about the GTX 600 and HD 7000 series cards.