Maarsch :
The i5 will see you right for gaming.
The added value for i7 is if you're into video/photo editing, streaming or the like.
There's just no 4.2 GHz i5 that fits in a Z170 motherboard. Unless you're planning on overclocking it to that level (and with a Z170 board you should be considering OCing).
Without knowing the exact model of CPU it is hard to say, but it might be worth getting a slightly lower tier CPU and pushing up your GPU a bit (R9 380 for a little bit more, GTX970 for a fair bit more)
INTEL Core i5-6600K, @ Taktfrequenz 4,20GHz, Quad-Core: "Skylake" • TDP: 95W • Fertigung: 14nm • Interface: DMI 3.0, 8GT/s • L2-Cache: 4x 256kB • L3-Cache: 6MB shared • Stepping: R0 • Einführung: 2015/Q3 • Grafik: Intel HD Graphics 530, 350-1150MHz, HDMI 1.4 + DisplayPort 1.2, max. 3 Displays • PCIe-Lanes: 16x PCIe 3.0 • Sockel: 1151, max. 1 CPU • Memory Controller: Dual Channel PC4-17000U/PC3L-12800U (DDR4-2133/DDR3L-1600), 34.1GB/s, max. 64GB • Features: SSE4.1, SSE4.2, AVX, AVX2, FMA3, Turbo Boost 2.0, VT-x EPT, VT-d, Intel 64, My WiFi, Idle States, EIST, Thermal Monitoring, IPT, AES-NI, TSX-NI, Secure Key, XD Bit, Clear Video HD, InTru 3D, Wireless Display,
Is what I plan on getting for a cpu
gpu
PALIT GeForce GTX 960 JetStream, 4GB GDDR5, 2x DVI, HDMI, DisplayPort, Chip: GM206-300-A1 "Maxwell" • Chiptakt: 1203MHz, Boost: 1266MHz • Speicher: 4GB GDDR5,7200MHz, 128bit, 115GB/s • Shader-Einheiten/TMUs/ROPs: 1024/64/32 • Rechenleistung: 2464GFLOPS (Single), 103GFLOPS (Double) • Fertigung: 28nm • Leistungsaufnahme: >120W (TDP), 10W (Leerlauf, gemessen) • DirectX: 12.0 • OpenGL: 4.4 • OpenCL: 1.2 • Shader Modell: 5.0 • Schnittstelle: PCIe 3.0 x16 • Kühlung: 2x Axial-Lüfter (100mm) • Besonderheiten: NVIDIA G-Sync, 2-Way-SLI, Zero-Fan-Modus, werkseitig übertaktet, HDCP 2.2
motherboard
MSI Z170-A PRO Mainboard, Chipsatz: Intel Z170 • Speicher: 4x DDR4 DIMM, dual PC4-25600U/DDR4-3200 (OC), max. 64GB (UDIMM) • Erweiterungsslots: 2x PCIe 3.0 x16 (1x x16, 1x x4), 4x PCIe 3.0 x1, 1x M.2/M-Key (PCIe 3.0 x4, 2280/2260/2242) • Anschlüsse extern: 1x VGA, 1x DVI-D, 1x DisplayPort 1.2, 4x USB 3.0, 2x USB 2.0, 1x Gb LAN (Realtek RTL8111H), 6x Klinke, 1x PS/2 Combo • Anschlüsse intern: 4x USB 3.0, 4x USB 2.0, 6x SATA 6Gb/s (Z170), 2x SATA Express (belegt 4x SATA, Z170), 2x CPU-Lüfter 4-Pin, 3x Lüfter 3-Pin, 1x seriell, 1x parallel, TPM-Header • Audio: 7.1 (Realtek ALC892) • RAID-Level: 0/1/5/10 (Z170) • Multi-GPU: AMD 2-Way-CrossFireX (x16/x4) • Stromanschlüsse: 1x 24-Pin ATX, 1x 8-Pin EPS12V • CPU-Phasen: 6 • Grafik: IGP (via CPU/APU)