Are you thinking triple monitors, or triple graphics cards?
Regardless anything more than dual GTX580 is not much worth it.
Get a second one now, and you will be able to play anything out there well for some time.
If and when you want more, just sell the GTX580's and get the next best thing.
It seems to me that the previous generation top end cards hold their value well.
If you now have a 2500K or better, there will be little value in sandy bridge -e/ Most games only use two or three fast cores, so a 6 core cpu will be half unused. I would wait for ivy bridge on the cpu side.