I3 has 2 cores, I5 has 4 cores, I7 has 4 cores plus 4 hyperthreading cores. 4+4 cores is going to blow away 2 cores in more things as more and more stuff has become multicore available.
People say the I7 is a waste for gaming because more games only use 2-4 cores so the extra 4 hyperthreading cores aren't used if you are just gaming, that extra $100 between an I5 and I7 could go for a better video card. If you are video encoding, editing, etc, then those extra 4 hyperthreading cores do come in handy.
Eitherway, in 2013, I wouldn't buy a dual core PC for anything.