The difference in performance between the newer Ivy Bridge and older Sandy Bridge assuming the same clockspeed is on average 6%. Over the past several years, benchmarks have shown that HyperThreading (in Core i7 CPUs) can decrease gaming performance by about 1% - 2% on average. Negligible. You can't tell the difference except by looking at benchmark numbers. You can always disable HT in the BIOS.
Net result = Sandy Bridge => Ivy Bridge = Waste of Money