Compare your motherboard to this wiki description of atx standard formats.
https://en.wikipedia.org/wiki/Computer_form_factor
Likely, it is standard.
It is cheaper to build today using standard parts.
I think you have only two ram slots available.
Check the specs on your particular pc to see how much ram is supported.
The ram you buy will NOT be the same.
Ram is sold in kits designed for a specific total capacity.
Changing the total capacity invalidates the original specs.
Buy only what a ram vendor says is compatible or is a compatible upgrade.
Otherwise, you have no recourse if it does not work.
If budget is a real issue, find a ram vendor recommended upgrade and look for the exact same part number on ebay; you might get it cheaper.