1.) The 955 and 965 are the same chip with the multiplier from the factory being the only difference. Both are Black Editions (unlocked) which means you can go into your BIOS and set the multiplier of the 955 to the 965 settings, or higher, for less cost.
2.) Not following this question.
3.) The i3 dual core series, but no way will it touch the processing power of a Sandy Bridge i5 or i7. Overclock the X4 to 4.0~ GHz and that gap narrows, but Sandy Bridge is still a faster processor.
4.) Either of those GPUs are a good choice. The HD 6870 is the better "bang for the buck".