Which GTS? If the original G80 GTS cards the GTX550TI is faster. If the newer G92 cards with 1GB of ram instead of 320/640MBs then the SLI setup is faster. But as you said you don't play intense games. And that single 550TI will use less power. I'd switch to the newer card.
I believe they are both 640 MBs. So you are saying the single gtx 550 ti is all around better?
I am a "noob" in terms of hardware and I never had a problem with overheating and I have no reason to believe my power supply is in any trouble. What are the benefits of using less power? ( I pay a flat fee for electricity )