|"750 Watt Power Supply"|
No make, model, or amperage specs?
What about the CPU heatsink? Did you get a new one or re-use the old? Did you thoroughly clean both mating surfaces & apply a tiny dab of thermal paste?
Also, you will most likely have to reinstall Vista due to the hardware change, so don't be surprised if the system won't boot into Windows. And out of curiousity, what did you upgrade from?