What do you have now?
cpu
gpu
motherboard
psu?
What are you trying to accomplish?
For example, sli is often not a good upgrade option.
You will win synthetic fps benchmarks, but there are some issues.
Some games do not support dual gpu.
You will be prone to stuttering and screen tearing.
Usually a single strong graphics card will play better.
m.2 is a size format.
By itself, it offers no advantages except size over a conventional 2.5" ssd.
If you are looking for m.2 pcie X 4 attachment, you will get faster sequential operations.
If you want to overclock, how close to the max do you need to get?
Most Z97 motherboards will respond well to any conservative overclock that your cpu chip will allow.