Your computer fits within the ATX spec, meaning it will take an ATX specification power supply. Not to worry though, there are a ton of them out there.
First of all, I would highly recommend the 7770 over the 550Ti. Its faster and about the same price as the 550Ti, and usually comes with a free game. You could also go for the 6850, which is even better.
Not all power supplies are made equally. Just like cars, some are made like crap, and some are made with quality workmanship and components.
Good companies are PC Power & Cooling, Corsair, Seasonic, and Antec. There's other ones, but those are the main renowned brands. http://www.newegg.com/Product/Product.aspx?Item=N82E16817139027&Tpk=corsair%20CX500
This is a good power supply if you are on a budget. One of the best there is.