We examine energy spectra of Si quantum dots embedded into Si_0.75Ge_0.25 buffers using atomistic numerical calculations for dimensions relevant to qubit implementations. The valley degeneracy of the lowest orbital state is lifted and valley splitting fluctuates with monolayer frequency as a function of the dot thickness. For dot thicknesses <6 nm valley splitting is found to be >150 ueV. Using the unique advantage of atomistic calculations we analyze the effect of buffer disorder on valley

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... tting. Disorder in the buffer leads to the suppression of valley splitting by a factor of 2.5, the splitting fluctuates with 20 ueV for different disorder realizations. Through these simulations we can guide future experiments into regions of low device-to-device fluctuations.