genthug :
Depends on the rest of your system mostly. I'm inclined to say no, but if you have a nice motherboard then you'll be able to overclock the 3570k. But, if you don't have a nice motherboard then you won't end up being able to overclock all that heavily and at that point it will be wasted money.
The 3450 can be overclocked too, with an OC-capable board.
The clock difference with OC is not going to be
that much bigger than the difference in the stock clocks.
OP, if you can sell the 3450 for $30-40 it's probably worth it.