unless you go with an amd s939 (opty 165 is prolly best), you will have to upgrade your ram in addition to your mobo, cpu and video card. in which case 400 bucks aint gonna cut it. the opteron can use your ddr ram. i think there is one asrock board that uses ddr for a c2d, but the mobo, cpu and card will add up quite a bit more than 400.00. plus, you just dont want to use ddr ram with a core 2 duo.
i dont think you will be able to upgrade to c2d for 400 bucks, sorry.
using the opty (its dual core also and a very good cpu), you can get a mobo, cpu, pci-e video card and probably a decent psu for very close to that amount. you may not need a psu if the 12v amperage is sufficient to run a pci-e video card.
if that strikes your fancy lemme know and i will post some newegg links.