Your processor is a 462 socket so it will not take a 754 or a 939 processor, without a new motherboard.
Your graphics card will be an AGP and although you can get an AGP 6600Gt card you will not notice that much of an improvement over your old set up.
The only way to improve is to get a new motherboard, processor & a PCIE graphics card.
It might be possible for £250 if you have a look on something like E-Bay.
Also what is your memory, is it 2100, 2700 or 3200?.