Your Pentium G3258 will undoubtedly bottleneck a GTX 1070; probably even a GTX 1060. However coffee lake is supposed to be announced in five days. I presume that it'll be for sale within 30 days or less from that date. I personally don't think that's a long time to with that CPU until you upgrade. Be sure to check the prices of the GTX 1080 models. If a GTX 1070 is $450 USD and a 1080 is $500, then the 1080 would be a better value.