I have a Asus Z87 Sabertooth mobo. It has 1-X16 PCIe slot, if I use the second slot they both become 2-X8 slots. I'm looking to upgrade my GPU. My qurstion is: 2- GTX 770 in SLI both @ X8 or 1- GTX 780ti @ X16 ?
1 780 ti is more than you need now. And since it is a gaming gpu, the prices will drop over time (as opposed to the titan). Therefore, it is probably you're best bet to buy the 780ti now and sli it in a couple of years.
Always get a single card less power draw and not all games are optimized for sli (some don't even work). gtx 780ti is all that you need right now, maybe even an overkill.