Need help choosing PSU

Yes, that PSU will work fine without any problems. It should have all the cables you need and provide enough power.

I want you to consider getting a more expensive, better PSU though; especially if you plan to overclock. That PSU ISN'T 80+ certified, has NO PFC, and won't give you much extra room since it's only 550W.

I urge you to consider the Corsair 650TX: http://www.newegg.com/Product/Product.aspx?Item=N82E16817139005

If you cannot budget for this PSU, at least try to find something with the following:
Active PFC
80+ Certified
500W (minimum)
Two (minimum) 6-pin (or 6+2-pin) PCI-e power cables

Rosewill is known for some bad PSUs.

Corsair, OCZ, Antec earthwatts, Seasonic would all be better manufacturers to look at.

Is there a reason you want twin 4770s (which would cost over $200) instead of a single 4870 or even 4890 for that price?
http://www.newegg.com/Product/Prod [...] 6814161294

OCZ 550W Fatality PSU (handles one 4870 or 4890 GPU or two 4770s) for $40 after $25 MIR.
http://www.newegg.com/Product/Prod [...] 6817341022

If you need isnt urgent, the new radeon 5770 should be coming within a month or so and would give you DX11 capability.

If you are going to spend that much to upgrade your graphics situation, you need to spend more time making money to upgrade the PSU properly. Don't skimp on the PSU. Stick with the brands DNDHATCHER mentioned and pay a bit more for good quality. It isn't worth upgrading the GPU while skimping the PSU because you increase the risk of killing your entire system. If anything, put priority on a stronger PSU than strong graphics.

Just my opinion.