1) No idea off the top of my head, but I would guess that first off, it would be silly, due to how cheap RAM is these days, and two, no, because the read/write speeds are so different.
2) Don't go with the 560 Ti, the 660 Ti will be out before too long and it will be much better. If you really want good performance, I suggest the 660, assuming you can afford the price tag.
3) Well, to get a new CPU, you would need a new mobo, since the socket type is 775 and you already have a quad core processor.