You can't replace that processor (which is for an LGA 775 socket) with an i5 without also replacing the motherboard. You might be able to replace it with a Core 2 Duo or Core 2 Quad without replacing the motherboard, depending on how limited the motherboard is, to answer that question you will need to know the make and model of the motherboard. You should be aware that old processors are surprisingly expensive to buy new at this time, it would probably make more sense to buy a modern processor and motherboard than a full price Core 2 Quad.
http://www.amazon.co.uk/s/ref=nb_sb_...therboard+-fan
The graphics card is also old, but I think the priority replacement is just about the CPU in this case.