well, that will depend on what socket motherboard you have and what you do,
If you have a 478 socket motherboard, well just like me, you'll have to do an entire computer upgrade.
If you have a LGA 775 motherboard, we will need to know what motherboard it is. Just so that if your motherboard would support a core 2 duo/quad.
Also what are you planing to do? web, word, gaming, rendering, ect.