It a little different for everybody, some CPUs overclock better. But stock vsel(56) and 1000Mhz works for many people, for me it caused random restars 1-2 times a day. 900Mhz is max for me, if i want 56vsel. And it realy is good to have 56 couse of battery drain.
So 800Mhz i think should work stable on 56 for everybody, just play a little with settings, you cant break anything, just phone will reset, if it will be unstable.
For example if you overclock with G.O.T. open recovery you will have 56vsel on any freq, including 1000Mhz. I remomand using for overclock, just put on SD card, apply setup.zip in recovery and than chose in menu to overclock to 800Mhz, done