an i7 would give you the best performance, but if you oc an e8x00 you'll be fine. and if you plan on tri-sli then get the 780i, if not a 750i will be fine. also get the evga 750i-FTW edition.
the 790i is a bit silly because it runs ddr3 in dual channel rather than tri channel which gives it better performance. so either get ddr2 and 750i ftw/78iftw or ddr3 x58