Core2 duos have different voltage requirements. I suggest a motherboard upgrade to a g31. Newegg has biostar open box g31 boards for $27.99 plus shipping. Biostar will send you a backplate for free if you email them.
I have the same board and recently found a pentium d 2.8g 800fsb processor on craigs list local to me and ind installed a radeon 4670 w/1gig video memory and 2 gig of system memory 750 gig hard drive sata all works well. I have one issue though, I installed a tv wonder hd650 pro tuner card and the computer failed to boot. Remove the card and it worked fine. Installed all vista updates, etc. Still same problem. I have a 500watt power supply. Could I be under powered?