3550 is not overclockable. and basically lower than 3570k. (although i believe it has some sort of "boost" option to get .4ghz more speed)
3570k is top of i5 ivy bridge right now, with "k" it is overclockable.
I know that. I'm saying on a budget rig the parts needed to properly overclock a 3570K would put the rig out of the OP's price range. The OP wants a build that's between $600 - $700, so a proper 3570K build would cost more like $900 - $1000.