A working 400W psu will power any system not using a video card. So unless you forgot to list your vid card, that's not the issue. 20 and 4 pin should work instead of 24 and 8 pin.
When was the last time this psu powered anything? Can you put it back and see that it still works in the old system?
- Try booting with just one stick of memory . . . then try with the other. If it fails to boot with either . . .
- Pull power/data from the HD & DVD and remove all memory. Power up. Does it power up? What do you hear from your onboard speaker?