So I have two questions, I am buying new computer parts to upgrade my computer
I am going to buy a COPY of Windows 10 and maybe ram if necessary.
My first question is will this ram (from a PREVIOUS build) be compatible with my motheboard?

RAM:
http://www.newegg.com/Product/Product.aspx?Item=N82E16820231671&nm_mc=AFC-C8Junction&cm_mmc=AFC-C8Junction-PCPartPicker,%20LLC-_-na-_-na-_-na&cm_sp=&AID=10446076&PID=3938566&SID=

Motherboard:
http://www.newegg.com/Product/Product.aspx?Item=N82E16813130731

Also I would like to know how to reuse the hard drive that's already in my computer, it currently has another OS on it and is running on the computer I am writing this message from. I don't know what the steps i have to take to reuse the hard drive and reinstal a new OSRAM is not a problem.
Your motherboard will support two hard drives.
You can, if you want to, buy a new hard drive like the one you have. Or even a smaller one.
In the computer BIOS there is some way to select which drive is the boot up drive.
If you buy the Windows 10 disk, you do not have to do the upgrade step. Unless you want to. Even then, you have options.
How To Physically Install A Second Internal Hard Drive

Of course, there has to be enough space inside the box for the second drive. If space is limited, you might pull out the floppy drive and not use it. Who ever does? Quote

If space is limited, you might pull out the floppy drive and not use it. Who ever does?

I doubt a system capable of running Windows 10 will typically have a Floppy Drive. For the best, though- Windows 10 literally doesn't support Floppy Drives at all. (OK, that isn't true- it doesn't support Floppy Drive Controllers, so you can still connect USB Floppies)

I tested a Win10 install on a machine that happened to have an FDD, Win10 File explorer saw the "A" drive.