Do you already own the 2500K? If so, definitely not worth it.
Compared with 2500K, 3570K is slightly (eg about 5%) faster at a given clock speed. But if the reports are to believed it also has slightly less overclocking headroom. Performance is pretty much equal.
If you are choosing for a new build, I would marginally recommend 3570K but only because it consumes marginally less power and because it is the current generation. 3570K is a straight replacement for the 2500K but in reality, hardly better if at all.
Yes, it will slot into your z68 mobo but you may need to update the bios.
For a new build, i would recommend i5 3570k for gaming. Note that you will have to put up with IB heating issues (if overclocked) besides relatively a higher temperature than the SB. You may not be able to overclock the IB processor as much as the SB.
If you plan to use any existing mobo, please check if it is compatible with IB. If it is, I reckon you need to upgrade the firmware using the existing CPU before replaciing it. If you do not have any z68 mobo, I would recommend a z77.
BTW, based on your post, you already seems to have a PC with i5 2500K. If yes, you do not gain anything. You may overclock the i5 2500K and wait for Haswell.