Unless your current gtx 750i is dead, the gtx 750 ti strix OC is the same thing

You should get something better. Getting the same thing is not worth it. Games gets more demanding over time.

lol its all down to ur initial budget, 2k? 4k? 6k? without a fix budget its impossible to recommend pc parts.

if u want sli, its easy, just get a sli rdy mobo and u r good to go 

gtx1070 is more than enough for 1080p gaming [for now], sli isnt a bad idea for future upgrade but u say its in 1~2 year, u might have a hard time to get a brand new/2nd hand same model 1070, different model might work but might have compatibility issues. and if u r getting a used gpu, the condition will be questionable.

Is better upgrade to new GPU instead of buy another unit for SLI.

The price of one GTX1070 is already 2k, after adding the processor that doesn't bottleneck it, a SLI-ready mobo, RAM, HDD/SSD, SLI-ready PSU, casing, it could easily go up to 5k+.

Planning to buy at the end of the year? Then your choice might not be the GTX1070 anymore if Nvidia releases their Volta based GTX card by end of this year (or perhaps you might switch to red team for the Vega card too).

And nope, don't ever plan to do SLI and hope it will perform better than a faster single card, SLI support from developers is going downhill in these recent years, either they totally abandon it or just have poor scaling, making the second card heavy underutilized in the long run. In conclusion, stick with the strongest & most powerful single card instead of SLI-ing the weaker cards, you will benefit more from doing so and potentially save more cost as well since you don't have to fork out more for a SLI-ready mobo and PSU initially.