1) The 750 is faster, have a look at
this review. It compares a bunch of chips in games, including the i5 750 and the E8600). In multi threaded apps, the i5 demolishes the dual core.
2)
Yes
3) Its governed by the chip, so yes.
4) A single Nvidia card will work fine. Obviously, you can't add a second in SLI (as it isn't supported), but one card will have no problems.