New Motherboard+CPU or New GPU?

What about wait a few days and discover if The witcher is more gpu or cpu dependent :P ?

Also, will a i7 870 bottleneck a gtx 970? No, that CPU is still a biggass CPU.

The 870 will bottleneck depending on the game and the resolution, though 1600*900 is outside of cpu-bottlenecking in most cases - likely, the CPU will bottleneck in strategy games and such, unlikely in TW3 or Arkham Knight.
Also, I'd wait for benchmarks on the games before upgrading, so you can adjust the system better to the conditions - I'd likely go for the GPU, regarding those two games.
In general though, there's a new CPU-line coming up in Q2/3 of this year, so I would'nt upgrade the CPU/Board now.

You should buy a new GPU if you want, o try to buy something in the second hand market if you don't want to pay so much and wait for next lineups of CPU and GPU, an AMD R9 280X/HD7970 or GTX 670/80 at a good price will improve your graphics about a 50% with no much money to expend, only about 30$ if you sell your GPU.

As Thallassa says, there's a new CPU-line, Intel Skylake, for the Q3 with new mobos, a 15% improvement expected, more PCIe lines for new and faster SSDs and DDR4, so, you should wait, even you can wait to change your GPU, because a new lineup of GPU is comming with AMD r9 390 or GTX 980Ti in the next months.

I'm sorry for my grammar, I'll try my best xD

your pc is fine, i can run most new games on my core duo with a gtx 550 with all settings maxed

You shouldn't need an upgrade at all for The Witcher 3. The graphics aren't as demanding as you'd thought. Though there's no reason to change your CPU.

Your current system seems to be within the minimum spec range for both Witcher 3 and Arkham Knight, so you don't really have to buy anything right now. As Caffettiera said, wait for reviews.

I think that your current system is sufficient productive for the games you mentioned at resolution 1600x900 at medium or high settings.
Your CPU is better than minimum requested for Witcher 3 and Arkham Knight, but GTX 660 is only minimum for both these games.
Both Witcher 3 and Arkham Knight use PhysX, and powerfull GPU from NVIDIA would be appropriate. That could be a reason to buy GTX 970.
Your CPU can limitate GPU in some games, for example, Assassin's Creed: Unity, but this is a common headache for all gamers ;(
By the way: you got a motherbord with notebook from DELL. How can it work with desktop CPU and GPU?

The higher the resolution and higher the detail settings, the more the GPU is the bottleneck. You'd be best off getting the 970 which would be a massive upgrade over your current GPU. Upgrading the CPU will do nothing to improve your FPS with a 660.

Thank you all for replying, you've been really helpful :)