Can i use 8 gb 1866 mhz to my HP pro 3500?

To use more than 4GB of memory, you'll need to perform a full, clean installation of a 64-bit version of Windows. 32-bit versions can't take advantage of more than 4GB. It is also not possible to simply upgrade from any 32-bit version of Windows to any 64-bit version of Windows.

Get whatever is cheaper. What specific models of memory are you considering?
 


 

Yes. Save your files/docs to an external device/cloud. Part of the install will be to format your primary storage device.

Either of the memory models should work. Given this is an OEM system, I would go with the cheaper of the 2 options.