Bought a z75 should i upgrade to z77?

I bought a z75 as rock with no sli. should i upgrade to a z77 with sli? what differences are there for bf3 will it run better on stock speed. Im just talking about the same hardware with a 660 ti superclocked will it make any difference? I dont plan on overclocing since its all to confusing. But i want sli and i got a new serial from ms ( i made some lies) (i have oem win 7) And i dont want to have to fill up my hdd with stuff and have to reinstall all of it myself