I agree with B_Z.
Sure you can get a Phenom II X4 965 (980 or 97x if you are really lucky), but it won't make much difference.Your CPU is already clocked at 3.6GHz; the Phenom II x4 980 is clocked at 3.7GHz...Your chip should be okay for a few more years.
If you really want to upgrade, you'd need to replace the aged 790FX board and go with a 900 series one (990X, 990FX), and get a 2nd generation FX chip (i.e 6300, 8320, 8350).
With the CPU aside, the Hd 7770 is the weakest link in your system. The Phenom II X4 955 @ 3.6GHz should be able to work fine with a HD 7870 / 7950 / 7970.
The forum layout is messed up for me, so I don't know of my post is set up the way I have it intended...(maybe browser related)
Phenom II X6 1090T @ 4.1GHz FX-8350 @ 4.8GHz // ASUS Crosshair V Formula 990FX // Sapphire Radeon HD Dual-X 7970 @ 1150/1500 // Thermaltake Frio w/ push-pull (using Antec Formula 7 Nano-Diamond + Cooler Master JetFlo fans) // 8GB (2 X 4GB) G.Skill RipJawsX 2133MHz // Corsair TX850 850W // Corsair Force GT 120GB SSD (OS) // Kingston V200+ 120GB SSD // WD Caviar Black 1TB // Windows 7 Ult x64