first your using ddr2 ram not ddr3 and 800mhz is perfectly fine i doubt ud notice an increase in using 1066mhz ram other than benchmarks.
And to your last question, if you buy from a manufacturer eg. dell than most likely they will give you the slower ddr2 667mhz, but if you were to buy a custom pre built pc than theres a good chance u will get 800mhz but if the person is stingy than they will skimp out and give you the 667mhz.