For starters DDR2 Ram is not compatible with a DDR3 Motherboard
If you change your Motherboard you will need ram to support it and possibly a new CPU also
Chances are he cannot source a new DDR2 Motherboard as they are obsolete - and a second hand one will not carry a warranty
Depending on what your old system is it may be time to just buy a new cheap computer as the cost to upgrade old tech and using old components can cost just as much.
Also depending on what you are using your computer for also is a factor - New can mean your old Printer, Scanner, Camera etc may just not work on a new OS - food for thought