The part number is F8098. Service manual: support.dell.com, enter service tag, choose Manuals, Service Manual, removing and replacing parts. Yes, it will void your warranty to replace the system board.
bearing in mind it is a 220watt PSU in the GX520 do you still stand by this statement? or would the 305w be needed? i see your test shows only 200w drawn but would extra not be needed to allow a tolerance for peaks?
Generally speaking, you can push DELL's power supplies. They are undermarked! Based on my tests 200.8W was the "highest" consumed (peak) with all devices FULL LOAD (Graphics Card, Processor, 2 x HD, 2 x DVD and Network Card). If you decide to go, don't forget to share your results later at this Forum.