A single 690 can even out perform 690 sli because when you put 690 into sli it means the game reads it as a Quad SLI which 80% of games don't support so it means the Frame rates go down pretty badly(the more GPU there is and if that game don't support multi GPu then the Frame rates will keep going lower)
Titan for a single screen @1080p? No, I don't think that's a particularly wise investment. You could buy a pair of 670's or something similiar and get much better value for money. Titan is mostly a marketing fad and as such is hopelessly overpriced.
To sli GTX 580 is not an option, mostly because of my power supply which barely has enough power for the task, but also availability of the card. It's totally unavailable here in Norway, have to import it then.
Sli Titan is not an option either since they are quite (insanely!) expensive.
Looks like at may be smart to wait a year? My card is still going strong, no doubt about that.
i know...the OP is pretty much maxed out as far as fps is concerned. even if he does go to a 690 hes not gonna see huge fps jumps. hes well above 30fps on every game with that rig now as it is.
the budget side of me says this is a frivolous upgrade.