For games that take advantage of a quad core you will see a big performance gain upgrading to an I5 or I7. The Difference between 8x and 16x is barely noticeable maybe about a 5% loss using 8x. As for the big water setup I dont know enough about that your best bet would be to check with the manufacturer and see if they sell a kit for the 1366 or 1156 boards.
So should you upgrade? I would say yes if you are serious about gaming and want to run SLI. The I5 750 will be plenty for gaming purposes and like I said the difference in performance between 8x and 16x is only about 5% loss.
Well if your asking if 2 GTX295s running 8x 8x is faster then 1 GTX295 running at 16x yes the 2 is faster then the 1. 2 GTX295s will rock like i said in my last post even though its running at 8x its only 5% slower then 16x yes 16x would be a little faster but no it is not a noticeable difference besides in benchmarks.