You will defiantly need a new PSU, i would advise a 750w Bronze or better. (If using the crossfire, if not then the one you picked is perfect)
As Long as the Chipset is the same it does not matter what brand Graphics card you purchase here, Newegg.com - SAPPHIRE 100326DDR3L Radeon HD 6670 1GB 128-bit DDR3 PCI Express 2.1 x16 HDCP Ready Video Card
Should Work Fine.
However, you could still go for the GTX 560 Ti If you wanted, and have the power of these two crossfired in one card, therefor in the future you can SLI.
The GTX 560 Ti is much more powerful than the 6670 and definatly more reliable than a crossfire. So if you have the money go for the GTX 560.
You picked good, you should really consider just building your own machine, alot cheaper and that way you get the specs you want