It is actually very easy to transfer your data. All you will need is a ATA to USB adapter which you can get from iFixit here:
[product|IF107-108]
and the guide to remove your hard drive:
[guide|1618]
From my knowledge it is probably the power supply but I cannot be for sure. The symptoms you describe are like that of a friends PowerMac MDD and they were solved after replacing the power supply. What are the last 3 digits of your serial number. This will help me locate the right power supply for you mac.