Same specs as above, but I will be getting a new PSU. Would it be best for me to go ahead and get the GTX 550, or should I get a Radeon equivalent since my mobo/CPU are AMD? Or does it not matter?
It doesn't really matter whether the brands are the same or not. It matters more your budget, resolution of gaming and preferred eye-candy. Then, you find the card which fits right into your requirements.
Though, I personally, like to match the third-party brands (eg. Asus, MSI, EVGA)
1. With that 600W PSU, you should be able to handle most GPUs. So no worries there.
2. What is your budget for the new card considering the above statement?