I would tell you that by the time you buy
all you plan the only thing you are not buying is an new tower and the
power pack. Firstly the new mother board may not fit in the older tower
and the power pack in it will be under powered, A new case is cheap with
at least a a power pack of at lease haf the power again and another
cable that powers your new mother board.

In your old system get rid of software you never use. The old
improvement is less junk and faster processing and more RAM.