Firstly you have to check what you main board can handle, if you have a manual then it should advise you to what CPU versions are available to install.
If you have the type and model of the main board then you can find this info on the website of the board brand.
Also check the cooling requirements.
Yes 8 core will be faster.
The later processor models will also run faster even if 4 core is selected.
I have a 12 Core I7 4930K 3400 MHz processor on a Asus Sabertooth X79 main board.
RAM speed is also important.
You can look at this site to see your processor speed compared to all the rest.
https://www.cpubenchmark.net/
I guess we will all want to update our PC's sooner or later.
If you need more help let us know.
Regards,
Mauri.