i found that +100 core 55% tdp and 400 mem is better than -502 mem and 55% tdp with 0 core, same wattage but more hashrate
You mean -400 or +400 mem Amph?
+400 but on a 1070 i was not talking about the 750
i found that +100 core 55% tdp and 400 mem is better than -502 mem and 55% tdp with 0 core, same wattage but more hashrate
+400 mem its not enough high ?
What is the limit for nvidia gtx 1070 gigabyte .
up to 600 it's ok, but above 600 not so much for certain 1070 models, there is risk of artifact
Not correct. Even +400 can lead to severe artifacts.
Better not to push beyond +300 for the gddr5x of the pascal architecture, regardless of the ram producer, and ns amounts.
i'm fine at 400, no artifact here, but i agree that overclocking mem is dangerous i don't like it for long terms
Mem O/C is affected by memory type, NVIDIA sneakily switched many newer 1070 cards to Micron 8GB GDDR5, which have BIOS issues and some of which can't do +400 . Samsung memory will usually hit +600 no problems. I think all RX 480 8GB cards use the same Samsung memory. IMO Overclocking should be fine if the memory is cooled properly, which is sometimes not the case like with the recent EVGA fiasco, the rated temperature of GDDR5 is like 85c.
Thanks for your answers guys, all your advice is welcome.
try to use all identical cards.
i'll go for a 1070, if you sell your 3 750Ti, that should pay for about 50% of what a 1070 costs.
that's what i did, i sell all my 750ti's (i had like 30

) and buy 1070's my power bill
decreased like 30% 
750ti where fantastic cards, the best ones i ever used, but their time for mining is over IMHO.
The number checks out.
On one hand there's the difference in fabrication process which says a lot:
GTX 750/750Ti/760/770/780/780Ti/Titan/Titan Z - 28nm
GTX 950/960/970/980/980 Ti/Titan X - 28 nm
GTX 1060/1070/1080/Titan X(P) - 16 nm
GTX 1050/1050 Ti - 14 nm
But, every card has their advantages and disadvantages (e.g. different efficiency) on a per algo basis.
Considering how old the 750 Ti's are, I'm still making ~70% of what I make with 1070's watt for watt.
But it's not as easy to sell them.
I think 1070 loses efficiency powering the 8GB of memory, the 1060 3GB should be best, except for in ethereum.