Its a bit of an oldish computer (from ~2007) if you want to upgrade the cpu you will need a new motherboard to support a newer cpu.
Most likely you will also need to upgrade the ram and the psu. My advice would be to save up for a new computer instead, as the graphics card in the vostro 200 is going to have a hard time running todays games etc.
EDIT: scottie is right of course my post was made from the point of view that it doesn't make much sense (to me) to put money into an old system. But yeah it should be possible to upgrade the cpu