The PCV-RS720G destop PC came with a proprietary 295W power Supply by Delta Electronics. Delta Electronics say they dont have a 400W power supply for my PCV-RS720G. Sony say the 295W Power Supply is the only one they have available for the PCV-RS720G. Some TechSpot Members have mentioned upgrading their PCV-RS720G to a 400W Power Supply. Anyone know where I can get a 400W power supply that fits in the Sony PCV-RS720G or one that can be made to fit?

Measure the existing power supply in length, width, and height in millimeters, then buy another the same size of 400 watts.
Delta is a pretty crummy power supply, so you should be able to get a better one by FSP group, Seasonic, Antec, or whatever.
There is a good "stickie" on this forum that discusses all the good and bad power supplies. You might want to give that a read. It is not complete, but it is mostly a very good list of problems and what works.

Yeah, I took the time to look up this P.C. cause I didn't know much about Vaio. Zalman makes very good PSU'S. So do Antec, etc.