The 965 is not worth the ~$25 over the 955. Get the 955 instead. Spend the money on a decent HSF (read: CM Hyper 212+) if you don't have one already.
Performance increase depends on what games you're playing. If you play newer games, you'll see a decent boost in performance. If there's any bottleneck in your rig ATM, it's most likely from the dual-core.
There are plenty of AM3 boards that support DDR3 1600. I suggest this one. It supports USB 3.0 and SATA III and x8/x8 CF.
For overclocking I guess I would go for the Asus M4A79XTD EVO but the one you mentioned is good too.
I don't think you'll get much more performance since in gaming the video card is much more important than the CPU and you already have a fast CPU. Upgrading to a quad will help you mainly in some newer games. And your RAM is actually faster than the DDR3 1333.