I was told that lower ratio comes with stability issue.
I'm not sure what you mean by this.
will it be more stable using ddr2 800 or ddr 2 1066
Assuming you are talking about memory bus speed, not DIMM ratings, then DDR2-800 would be at least as stable as DDR2-1066. The stability of DDR2-1066 speed would depend on the quality of the RAM you are using.