Gtx 670 sli

I'm currently running a GTX 670 to power my BenQ XL2420TX. I was wondering if it would be worth getting another one for when new games become more graphically demanding or if I should just wait. Perhaps it would be better purchasing a higher series card further down the line?

I'm only running a 600W power supply, so I would have to upgrade that to run SLI.

just go outside bro, that has the best  graphics im told.

what new games need you to sli a gtx 670?

I was thinking of games like Star Citizen that are supposed to be "unplayable" on current specs (at least at Ultra settings). 

P.s. I live in the UK, the graphics outside aren't that great this time of year.