i mined on windows 7 64 bit and now Ubuntu 11.04 32 bit.

there is no reall difference, perhaps ubuntu hast +1 MH/s on my 6950.
the only difference is, that with actual drivers on windows you cannot use hardware acceleration if you're using a amd 6950 (maybe the whole 6xxx series), because the system freezes.

instead of trying to reach maximum MH/s you should try to reach maximum uptime. both your pc and mining pool.