He's basically saying that the quality of your power supply is very poor in comparison to the rest of your build. You have a cheap value unit Power Supply Unit which isn't optimal. But agreed yes your computer will not support SLi and I would just add that a 660Ti would be a better investment then a 560Ti at the moment in SLi. IF you are getting acceptable frames playing the games you are you are probably better off waiting for the 760ti's or possibly the 800 series cards after that. 2-3 generations is a good point of upgrade normally.
Yup which is always a good incentive to get quality power supply units. You should never ever skimp on one. Typically I find spending in the ballpark of 80-150 dollars on a power supply a worthwhile investment. For example here is a great power supply that would support sli. At least for the majority of what you would need to run a multi card configuration.
I suppose it all really depends. You could pcpartpicker anything really and get the best deal in all honesty. And for me newegg is literally 15-30 minutes away well there warehouse is that they ship from normally in California so I like receiving my item as quickly as I do. And I have preferred account so when I spend over 250 I get 6 months to pay it off and with 500 a year so not to shabby. They aren't the best always with prices but they are a pretty great company to purchase from. NCIX is good as well.