Are you talking about mixing the OCZ and Kingston modules? Or are the Kingstons in a separate machine, which you're also hoping to bump up?
1200 mhz is extremely ambitious for 800mhz. They'd have to be really good 'binned' modules to make that. It's highly unlikely you'll get 1200mhz. 900mhz is a safer and more realistic goal.
Also, with 6 GB on that board (P43-ES3G), you aren't running in dual-channel - this hurts performance to begin with.
However, If you want to try, the general guide is looser timings + more volts for RAM and the sometimes the Northbridge (NB)
Realistically though, with those specs, the difference would not be noticeable.