GPU: HD 7770
This is the fastest card I ever had and I'm very contented with it. I can play almost every game I have, though it is not the fastest card today, I think I still can use it for 2-3 years more.
go for the Radeon 7950 first since video card have a great impact on gaming performance, your motherboard and cpu are just fine they can perform well. If you can upgrade them both they will have great impact on your system since you have a Z77 board which support 3rd generation cpu and PCI 3.0...