Hello Thomas, I have the same motherboard as you (thanks for the PM) and maybe the same OEM PCV-RS520?
I was in the same predicament as you, wanting to add hardware and needed to find help. Same as you, I also recieved no help from Sony. They were complete a$$holes about my PSU questions. Not to mention how they hung up on me, and total rudeness is not exceptable for a customer. Needless to say I will never buy from them again. Unfortunately, that wasn't the only problem I had. Upgrading my PSU also required that I get a new case, because Sony uses proprietary parts, which means when it's time to upgrade...guess who gets the money? SONY! Well I'm sorry, I wasn't falling for thier marketing tactics. So I bought the case and PSU you see in my sig.
Let me give you another piece of advise. Be prepared to do battle with your Sony case. It wont fit any normal PSU, NO! Thats the cause of my upgrade move. Removing things from the case was a hassle, no, a royal pain in the a$$! Believe me, if you could see what I did to my case to be able to remove everything...well ....let's just say it doesn't resemble a case anymore lol.
Gaining independance from OEM manufacturers is the goal here Thomas, make no mistake about it.
My suggestion to you is this:
Buy a new case (gives you upgrade paths that were previously non-existant)
Buy a new PSU (Any PSU with higher wattage will work, Some come with 20+4 pin motherboard connectors, but all you need is the 20, just disconnect the +4 pin and your all set)
You could kill both birds with one stone (like I did) but I wouldn't recommend what I decided on buying. My PSU still limits me, and the case is louzy construction.
PSU recommendations: any Forton Source, Antec, Xclio.
Go with one of these three, they will give you more power than you need so you may still use the same PSU when you go to PCIe-->
Forton Source 450w
Antec SmartPower 2.0 500w
Xclio 480w
Remember, a high wattage rating doesn't mean it's the best. Something rated at 500w and costs $30 probly isn't going to do so well. Check the 12v rail, make sure it's atleast 30, to allow upgrades in the future. The one's I recommended have dual 12v rails and add up to over the 30 mark
As for a case, I would go with a well known manufacturer if I had to do it over. Antec, coolermaster, lian li, thermaltake are all decent with thier cases.
As for your question about voltage on the motherboard, I don't think that comes into play here. As long as you get a better PSU than you have, there should be no problems
PS. Feel free to PM me at anytime if you have any questions.