#1 Check benchmarks for performance increases on the i7. Depending on what you use your computer for, it could be a considerable game.
#2 AH can only use two cores at once. If the clock speed of the dual core and quad core were the same (and we're not talking different generation of CPU's), performance theoretically should be the same.
#3 I'm not positive. The GTX 295 is basically two cards on one physical card. I don't believe its considered 'SLI' if you only have 1 of these guys.