Ideally you want to wait for Pascal if you’re planning on getting a high end card since the graphics memory standard is switching over to the much faster HBM (high bandwidth memory), rather than the much older GDDR5.
From my understanding, the new Titan and 1080 (if that’s what it’s called) will feature HBM2, the 1070 might, and anything lower will not.
The issue is that no one really knows when they’re coming out. This report is far from confirmed, but no other reports are confirmed either.
Also, unless you REALLY need the graphics power I would avoid SLI. I went against my own advice this time around and got 2x980s, as did @Vocino, and we both regretted it. A lot of games/applications don’t even support SLI, so half the time you’re left a very expensive airflow restriction in your case. Not to mention it’s SUPER loud, generates way more heat, and draws way more power. Not worth it IMO.